pgsql: When two base backups are started at the same time with pg_baseb

When two base backups are started at the same time with pg_basebackup,
ensure that they use different checkpoints as the starting point. We use
the checkpoint redo location as a unique identifier for the base backup in
the end-of-backup record, and in the backup history file name.

Bug spotted by Fujii Masao.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/6d8096e2f3f2c1296fa880f44f3fa5701b2f40c4

Modified Files
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src/backend/access/transam/xlog.c |   95 +++++++++++++++++++++++-------------
1 files changed, 61 insertions(+), 34 deletions(-)