Avoid LWLockWaitForVar() for currently held WAL insertion lock in WaitXLogInsertionsToFinish()

Hi,

While working on something else, I noticed that
WaitXLogInsertionsToFinish() goes the LWLockWaitForVar() route even
for a process that's holding a WAL insertion lock. Typically, a
process holding WAL insertion lock reaches
WaitXLogInsertionsToFinish() when it's in need of WAL buffer pages for
its insertion and waits for other older in-progress WAL insertions to
finish. This fact guarantees that the process holding a WAL insertion
lock will never have its insertingAt less than 'upto'.

With that said, here's a small improvement I can think of, that is, to
avoid calling LWLockWaitForVar() for the WAL insertion lock the caller
of WaitXLogInsertionsToFinish() currently holds. Since
LWLockWaitForVar() does a bunch of things - holds interrupts, does
atomic reads, acquires and releases wait list lock and so on, avoiding
it may be a good idea IMO.

I'm attaching a patch herewith. Here's the cirrus-ci tests -
https://github.com/BRupireddy/postgres/tree/avoid_LWLockWaitForVar_for_currently_held_wal_ins_lock_v1.

Thoughts?

-- 
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com