pgsql: Use callbacks in SlruScanDirectory for the actual action

Use callbacks in SlruScanDirectory for the actual action

Previously, the code assumed that the only possible action to take was
to delete files behind a certain cutoff point. The async notify code
was already a crock: it used a different "pagePrecedes" function for
truncation than for regular operation. By allowing it to pass a
callback to SlruScanDirectory it can do cleanly exactly what it needs to
do.

The clog.c code also had its own use for SlruScanDirectory, which is
made a bit simpler with this.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/09e196e4539a70c51e828abcfe48dee3efd312d8

Modified Files
--------------
src/backend/access/transam/clog.c | 2 +-
src/backend/access/transam/slru.c | 104 +++++++++++++++++++++++++++----------
src/backend/commands/async.c | 36 +++----------
src/include/access/slru.h | 11 ++++-
4 files changed, 94 insertions(+), 59 deletions(-)