Our use of ps.Map implies many heap allocations

[bboreham]

Take StringLatestMap which is like map[string]*struct{time.Time,string}

Each node in the map is an allocation of a ps.tree struct (size 112 bytes).
Each key and value string is an allocation, and the value struct is an allocation because it's boxed to be held as an interface{} by the ps.tree struct

So a map with n entries requires 4n memory allocations.

Relates to #1010

I have started a reimplementation of StringLatestMap based on a slice, which is a similar idea to #1758.

[bboreham]

Another thought strikes me: each ps.tree.struct node has 8 pointers to other nodes and none of them is ever nil; they are initialised to point to nilMap. So garbage-collection of a ps.Map likely involves a lot of chasing down those pointers.

[bboreham]

By now, most ps.Maps were replaced by slices. The remaining ones:

• counters - not set in probe; only used by parent nodes in app
• sets - done in WIP Re-implement Sets as sorted slice, for performance #2962; needs a bit more work to finish
• node_set - replace by slice was more expensive, perhaps because node is large and we get large node_sets in specific cases such as Endpoints.