pgsql: Make pg_rewind skip files and directories that are removeddurin

Make pg_rewind skip files and directories that are removed during server start.

The target cluster that was rewound needs to perform recovery from
the checkpoint created at failover, which leads it to remove or recreate
some files and directories that may have been copied from the source
cluster. So pg_rewind can skip synchronizing such files and directories,
and which reduces the amount of data transferred during a rewind
without changing the usefulness of the operation.

Author: Michael Paquier
Reviewed-by: Anastasia Lubennikova, Stephen Frost and me

Discussion: https://postgr.es/m/20180205071022.GA17337@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/266b6acb312fc440c1c1a2036aa9da94916beac6

Modified Files
--------------
doc/src/sgml/ref/pg_rewind.sgml      |  14 +++-
src/backend/replication/basebackup.c |   3 +
src/bin/pg_rewind/filemap.c          | 144 ++++++++++++++++++++++++++++++++---
3 files changed, 148 insertions(+), 13 deletions(-)