Fix corruption of pgstats shared hashtable due to OOM failures

Fix corruption of pgstats shared hashtable due to OOM failures

A new pgstats entry is created as a two-step process:
- The entry is looked at in the shared hashtable of pgstats, and is
inserted if not found.
- When not found and inserted, its fields are then initialized.  This
part include a DSA chunk allocation for the stats data of the new entry.

As currently coded, if the DSA chunk allocation fails due to an
out-of-memory failure, an ERROR is generated, leaving in the pgstats
shared hashtable an inconsistent entry due to the first step, as the
entry has already been inserted in the hashtable.  These broken entries
can then be found by other backends, crashing them.

There are only two callers of pgstat_init_entry(), when loading the
pgstats file at startup and when creating a new pgstats entry.  This
commit changes pgstat_init_entry() so as we use dsa_allocate_extended()
with DSA_ALLOC_NO_OOM, making it return NULL on allocation failure
instead of failing.  This way, a backend failing an entry creation can
take appropriate cleanup actions in the shared hashtable before throwing
an error.  Currently, this means removing the entry from the shared
hashtable before throwing the error for the allocation failure.

Out-of-memory errors unlikely happen in the wild, and we do not bother
with back-patches when these are fixed, usually.  However, the problem
dealt with here is a degree worse as it breaks the shared memory state
of pgstats, impacting other processes that may look at an inconsistent
entry that a different process has failed to create.

Author: Mikhail Kot <mikhail.kot@databricks.com>
Discussion: https://postgr.es/m/CAAi9E7jELo5_-sBENftnc2E8XhW2PKZJWfTC3i2y-GMQd2bcqQ@mail.gmail.com
Backpatch-through: 15