pgsql: Properly remove ephemeral replication slots after a crash restar

Properly remove ephemeral replication slots after a crash restart.

Ephemeral slots - slots that shouldn't survive database restarts -
weren't properly cleaned up after a immediate/crash restart. They were
ignored in the sense that they weren't restored into memory and thus
didn't cause unwanted resource retention; but they prevented a new
slot with the same name from being created.

Now ephemeral slots are fully removed during startup.

Backpatch to 9.4 where replication slots where added.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/fece13d7477720bcbf02af795bed695c54d1250c

Modified Files
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src/backend/replication/slot.c | 22 ++++++++++++++++++----
1 file changed, 18 insertions(+), 4 deletions(-)