pgsql: Clamp autovacuum launcher sleep time to 5 minutes

Clamp autovacuum launcher sleep time to 5 minutes

This avoids the problem that it might go to sleep for an unreasonable
amount of time in unusual conditions like the server clock moving
backwards an unreasonable amount of time.

(Simply moving the server clock forward again doesn't solve the problem
unless you wake up the autovacuum launcher manually, say by sending it
SIGHUP).

Per trouble report from Prakash Itnal in
https://www.postgresql.org/message-id/CAHC5u79-UqbapAABH2t4Rh2eYdyge0Zid-X=Xz-ZWZCBK42S0Q@mail.gmail.com

Analyzed independently by Haribabu Kommi and Tom Lane.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/cf733760eae1523022b652d4e673d336ef81b235

Modified Files
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src/backend/postmaster/autovacuum.c | 10 ++++++++++
1 file changed, 10 insertions(+)