pgsql: Write exact xlog position of timeline switch in the timeline his

Write exact xlog position of timeline switch in the timeline history file.

This allows us to do some more rigorous sanity checking for various
incorrect point-in-time recovery scenarios, and provides more information
for debugging purposes. It will also come handy in the upcoming patch to
allow timeline switches to be replicated by streaming replication.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/32f4de0adfb2037f1402e40b54a5c4043227363f

Modified Files
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src/backend/access/transam/timeline.c | 144 +++++++++++++++++++++++-----
src/backend/access/transam/xlog.c | 173 ++++++++++++++++++++------------
src/include/access/timeline.h | 20 ++++-
3 files changed, 247 insertions(+), 90 deletions(-)