pgsql: Fix an issue in recent walwriter hibernation patch.

Fix an issue in recent walwriter hibernation patch.

Users of asynchronous-commit mode expect there to be a guaranteed maximum
delay before an async commit's WAL records get flushed to disk. The
original version of the walwriter hibernation patch broke that. Add an
extra shared-memory flag to allow async commits to kick the walwriter out
of hibernation mode, without adding any noticeable overhead in cases where
no action is needed.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/acd4c7d58baf09fc278a0f8c1170fba2d56ccdd5

Modified Files
--------------
src/backend/access/transam/xlog.c | 48 ++++++++++++++++++++++++++++++-----
src/backend/postmaster/walwriter.c | 18 +++++++++++++
src/include/access/xlog.h | 1 +
3 files changed, 60 insertions(+), 7 deletions(-)