pgsql: Split off functions related to timeline history files and XLOG a

Split off functions related to timeline history files and XLOG archiving.

This is just refactoring, to make the functions accessible outside xlog.c.
A followup patch will make use of that, to allow fetching timeline history
files over streaming replication.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/d5497b95f3ca2fc50c6eef46d3394ab6e6855956

Modified Files
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src/backend/access/transam/Makefile      |    3 +-
src/backend/access/transam/timeline.c    |  378 ++++++++++++
src/backend/access/transam/xlog.c        |  988 ++----------------------------
src/backend/access/transam/xlogarchive.c |  572 +++++++++++++++++
src/include/access/timeline.h            |   23 +
src/include/access/xlog_internal.h       |   23 +
6 files changed, 1058 insertions(+), 929 deletions(-)