Fix ordering issue with WAL operations in GIN fast insert path

Fix ordering issue with WAL operations in GIN fast insert path

Contrary to what is documented in src/backend/access/transam/README,
ginHeapTupleFastInsert() had a few ordering issues with the way it does
its WAL operations when inserting items in its fast path.

First, when using a separate list, XLogBeginInsert() was being always
called before START_CRIT_SECTION(), and in this case a second thing was
wrong when merging lists, as an exclusive lock was taken on the tail
page *before* calling XLogBeginInsert().  Finally, when inserting items
into a tail page, the order of XLogBeginInsert() and
START_CRIT_SECTION() was reversed.  This commit addresses all these
issues by moving the calls of XLogBeginInsert() after all the pages
logged are locked and pinned, within a critical section.

This has been applied first only on HEAD as of 56b6625, but as per
discussion with Tom Lane and Álvaro Herrera, a backpatch is preferred to
keep all the branches consistent and to respect the transam's README
where we can.

Author:  Matthias van de Meent, Zhang Mingli
Discussion: https://postgr.es/m/CAEze2WhL8uLMqynnnCu1LAPwxD5RKEo0nHV+eXGg_N6ELU88HQ@mail.gmail.com
Backpatch-through: 10