pgsql: Improve generation algorithm for database system identifier.

Improve generation algorithm for database system identifier.

As noted some time ago, the original coding had a typo ("|" for "^")
that made the result less unique than intended. Even the intended
behavior is obsolete since it was based on wanting to produce a
usable value even if we didn't have int64 arithmetic --- a limitation
we stopped supporting years ago. Instead, let's redefine the system
identifier as tv_sec in the upper 32 bits (same as before), tv_usec
in the next 20 bits, and the low 12 bits of getpid() in the remaining
bits. This is still hardly guaranteed-universally-unique, but it's
noticeably better than before. Per my proposal at
<29019(dot)1374535940(at)sss(dot)pgh(dot)pa(dot)us>

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/5035701e07e8bd395aa878465a102afd7b74e8c3

Modified Files
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src/backend/access/transam/xlog.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)