integration: add more lease tests

[gyuho]

Fix #6102.

[xiang90]

Can you try to run these test before commit #6098?

[xiang90]

define the 5 * 60 thing as a 5minTTL as a global var in this test file?

[xiang90]

LGTM

[gyuho]

@xiang90 Without your Fix Lease PR, it fails:

go test -v -run TestV3LeaseRecoverKeyWithDetachedLease
=== RUN   TestV3LeaseRecoverKeyWithDetachedLease
--- FAIL: TestV3LeaseRecoverKeyWithDetachedLease (0.16s)
    v3_lease_test.go:530: only detached lease removed, key should remain
FAIL
exit status 1
FAIL    github.com/coreos/etcd/integration  0.169s
gyuho@gyuho: ~/go/src/github.com/coreos/etcd/integration $
go test -v -run TestV3LeaseRecoverKeyWithMutipleLease
=== RUN   TestV3LeaseRecoverKeyWithMutipleLease
--- FAIL: TestV3LeaseRecoverKeyWithMutipleLease (0.16s)
    v3_lease_test.go:589: only detached lease removed, key should remain

And we need to create a new client after restart because gRPC fails fast (we don't pass FailFast(false) in integration test). I added comments on this.

Will merge after greens, if there's no objection.

Thanks!