pgsql: Log when a BRIN autosummarization request fails

Log when a BRIN autosummarization request fails

Autovacuum's 'workitem' request queue is of limited size, so requests
can fail if they arrive more quickly than autovacuum can process them.
Emit a log message when this happens, to provide better visibility of
this.

Backpatch to 10.  While this represents an API change for
AutoVacuumRequestWork, that function is not yet prepared to deal with
external modules calling it, so there doesn't seem to be any risk (other
than log spam, that is.)

Author: Masahiko Sawada
Reviewed-by: Fabrízio Mello, Ildar Musin, Álvaro Herrera
Discussion: https://postgr.es/m/CAD21AoB1HrQhp6_4rTyHN5kWEJCEsG8YzsjZNt-ctoXSn5Uisw@mail.gmail.com

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/eadcb7a2377a7a68dee24f750b61a2ac0d7f9f40

Modified Files
--------------
doc/src/sgml/brin.sgml              | 12 ++++++++++++
src/backend/access/brin/brin.c      | 16 +++++++++++++---
src/backend/postmaster/autovacuum.c |  7 ++++++-
src/include/postmaster/autovacuum.h |  2 +-
4 files changed, 32 insertions(+), 5 deletions(-)