pgsql: Add a hook for processing messages due to be sent to the server

Add a hook for processing messages due to be sent to the server log.

Use-cases for this include custom log filtering rules and custom log
message transmission mechanisms (for instance, lossy log message
collection, which has been discussed several times recently).

As is our common practice for hooks, there's no regression test nor
user-facing documentation for this, though the author did exhibit a
sample module using the hook.

Martin Pihlak, reviewed by Marti Raudsepp

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/19dbc3463161a142537ba5c569c8e6a073a318de

Modified Files
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src/backend/utils/error/elog.c | 26 ++++++++++++++++++++++++++
src/include/utils/elog.h | 4 ++++
2 files changed, 30 insertions(+), 0 deletions(-)