panic in proto merge when deep-copying server value

[a-palchikov]

Description

What happened:
Very often, local integrations tests fail with:

panic: merger not found for type:int [recovered]
	panic: merger not found for type:int

goroutine 26 [running]:
testing.tRunner.func1.1(0xecb400, 0xc0003d43f0)
	/home/deemok/dev/go/go/src/testing/testing.go:1072 +0x30d
testing.tRunner.func1(0xc000103080)
	/home/deemok/dev/go/go/src/testing/testing.go:1075 +0x41a
panic(0xecb400, 0xc0003d43f0)
	/home/deemok/dev/go/go/src/runtime/panic.go:969 +0x1b9
github.com/gogo/protobuf/proto.(*mergeInfo).computeMergeInfo(0xc0003daa80)
	/home/deemok/go/src/github.com/gravitational/teleport-opt/vendor/github.com/gogo/protobuf/proto/table_merge.go:664 +0xfff
github.com/gogo/protobuf/proto.(*mergeInfo).merge(0xc0003daa80, 0xc0002eb760, 0xc0002267e0)
	/home/deemok/go/src/github.com/gravitational/teleport-opt/vendor/github.com/gogo/protobuf/proto/table_merge.go:113 +0x4d7
github.com/gogo/protobuf/proto.(*mergeInfo).computeMergeInfo.func27(0xc0003f27f0, 0x16e56b0)
	/home/deemok/go/src/github.com/gravitational/teleport-opt/vendor/github.com/gogo/protobuf/proto/table_merge.go:545 +0x386
github.com/gogo/protobuf/proto.(*mergeInfo).merge(0xc0003daa00, 0xc0003f27e0, 0x16e56a0)
	/home/deemok/go/src/github.com/gravitational/teleport-opt/vendor/github.com/gogo/protobuf/proto/table_merge.go:139 +0x49f
github.com/gogo/protobuf/proto.(*mergeInfo).computeMergeInfo.func30(0xc0002eb650, 0xc0002eb4b0)
	/home/deemok/go/src/github.com/gravitational/teleport-opt/vendor/github.com/gogo/protobuf/proto/table_merge.go:587 +0x5c
github.com/gogo/protobuf/proto.(*mergeInfo).merge(0xc0003da980, 0xc0002eb640, 0xc0002eb4a0)
	/home/deemok/go/src/github.com/gravitational/teleport-opt/vendor/github.com/gogo/protobuf/proto/table_merge.go:139 +0x49f
github.com/gogo/protobuf/proto.(*mergeInfo).computeMergeInfo.func30(0xc0003c6a68, 0xc0003c67e8)
	/home/deemok/go/src/github.com/gravitational/teleport-opt/vendor/github.com/gogo/protobuf/proto/table_merge.go:587 +0x5c
github.com/gogo/protobuf/proto.(*mergeInfo).merge(0xc0003da8c0, 0xc0003c6a30, 0xc0003c67b0)
	/home/deemok/go/src/github.com/gravitational/teleport-opt/vendor/github.com/gogo/protobuf/proto/table_merge.go:139 +0x49f
github.com/gogo/protobuf/proto.(*mergeInfo).computeMergeInfo.func28(0xc0003c6a30, 0xc0003c67b0)
	/home/deemok/go/src/github.com/gravitational/teleport-opt/vendor/github.com/gogo/protobuf/proto/table_merge.go:555 +0x3e
github.com/gogo/protobuf/proto.(*mergeInfo).merge(0xc0003da380, 0xc0003c6a00, 0xc0003c6780)
	/home/deemok/go/src/github.com/gravitational/teleport-opt/vendor/github.com/gogo/protobuf/proto/table_merge.go:139 +0x49f
github.com/gogo/protobuf/proto.(*InternalMessageInfo).Merge(0x16e46a0, 0x1140fc0, 0xc0003c6a00, 0x1140fc0, 0xc0003c6780)
	/home/deemok/go/src/github.com/gravitational/teleport-opt/vendor/github.com/gogo/protobuf/proto/table_merge.go:50 +0x4c
github.com/gravitational/teleport/api/types.(*ServerV2).XXX_Merge(0xc0003c6a00, 0x1140fc0, 0xc0003c6780)
	/home/deemok/go/src/github.com/gravitational/teleport-opt/vendor/github.com/gravitational/teleport/api/types/types.pb.go:580 +0x57
github.com/gogo/protobuf/proto.Merge(0x1140fc0, 0xc0003c6a00, 0x1140fc0, 0xc0003c6780)
	/home/deemok/go/src/github.com/gravitational/teleport-opt/vendor/github.com/gogo/protobuf/proto/clone.go:95 +0x346
github.com/gogo/protobuf/proto.Clone(0x1140fc0, 0xc0003c6780, 0x100b5b1, 0x5)
	/home/deemok/go/src/github.com/gravitational/teleport-opt/vendor/github.com/gogo/protobuf/proto/clone.go:52 +0x19b
github.com/gravitational/teleport/api/types.(*ServerV2).DeepCopy(...)
	/home/deemok/go/src/github.com/gravitational/teleport-opt/vendor/github.com/gravitational/teleport/api/types/server.go:351
...

The changes I made to mitigate another server test race by patching gogo implementation of proto.Clone does not seem to have the necessary effect.
The issue at hand is described here.

I'm going to rewrite the deep copy code without proto.Clone as it seems to be entirely unsuited for our needs.

What you expected to happen:
No panic.

Reproduction Steps

go test -count=1 ./integration/...