pgsql: Wake WALSender to reduce data loss at failover for async commit.

Wake WALSender to reduce data loss at failover for async commit.
WALSender now woken up after each background flush by WALwriter, avoiding
multi-second replication delay for an all-async commit workload.
Replication delay reduced from 7s with default settings to 200ms, allowing
significantly reduced data loss at failover.

Andres Freund and Simon Riggs

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/16222f32ed56d3ebc4136133662d932299188955

Modified Files
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src/backend/access/transam/xlog.c |    9 +++++++++
1 files changed, 9 insertions(+), 0 deletions(-)