pgsql: Tolerate timeline switches while "pg_basebackup -X fetch" is run

Tolerate timeline switches while "pg_basebackup -X fetch" is running.

If you take a base backup from a standby server with "pg_basebackup -X
fetch", and the timeline switches while the backup is being taken, the
backup used to fail with an error "requested WAL segment %s has already
been removed". This is because the server-side code that sends over the
required WAL files would not construct the WAL filename with the correct
timeline after a switch.

Fix that by using readdir() to scan pg_xlog for all the WAL segments in the
range, regardless of timeline.

Also, include all timeline history files in the backup, if taken with
"-X fetch". That fixes another related bug: If a timeline switch happened
just before the backup was initiated in a standby, the WAL segment
containing the initial checkpoint record contains WAL from the older
timeline too. Recovery will not accept that without a timeline history file
that lists the older timeline.

Backpatch to 9.2. Versions prior to that were not affected as you could not
take a base backup from a standby before 9.2.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/b0daba57bb50ca8bfb9d3ec886c5f86f7b3c006b

Modified Files
--------------
src/backend/access/transam/xlog.c | 23 +++-
src/backend/replication/basebackup.c | 231 ++++++++++++++++++++++++++++------
src/backend/replication/walsender.c | 8 +-
src/include/access/xlog.h | 2 +-
4 files changed, 211 insertions(+), 53 deletions(-)