Allocate thread-local snapshot array statically

Allocate thread-local snapshot array statically

Since commit fb56418d66 the snapshots are computed in thread-local
storage, but we haven't been freeing the memory (on thread exit).
As the memory is allocated in the global (TopMostMemoryContext),
this presented a memory leak of 64kB on each GTM connection.

One way to fix this would be to track when the thread-local storage
is used in GTM_GetTransactionSnapshot(), and allocate the memory
in TopMemoryContext (which is per-thread and released on exit).

But there's a simpler way - allocate the thread-specific storage as
part of GTM_ThreadInfo, and just redirect sn_xip from the snapshot.
This way we don't have to worry about palloc/pfree at all, and we
mostly assume that every connection will need to compute at least
one snapshot anyway.

Reported by Rob Canavan <rxcanavan@gmail.com>, investigation and fix
by me. For more discussion see
<CAFTg0q6VC_11+c=Q=gsAcDsBrDjvuGKjzNwH4Lr8vERRDn4Ycw@mail.gmail.com>

Backpatch to Postgres-XL 9.5.