Re: Issue in GIN fast-insert: XLogBeginInsert + Read/LockBuffer ordering

 In general, this works fine, except that in ginHeapTupleFastInsert we
call XLogBeginInsert() before the last of the buffers for the eventual
record was read, thus creating a path where eviction is possible in a
`begininsert_called = true` context. That breaks our current code by
being unable to evict (WAL-log) the dirtied hint pages.

PFA a patch that rectifies this issue, by moving the XLogBeginInsert()
down to where 1.) we have all relevant buffers pinned and locked, and
2.) we're in a critical section, making that part of the code
consistent with the general scheme for XLog insertion.