pgsql: Pre-pad WAL files when streaming transaction log

Pre-pad WAL files when streaming transaction log

Instead of filling files as they appear, pre-pad the
WAL files received when streaming xlog the same way
that the server does. Data is streamed into a .partial
file which is then renamed()d into palce when it's complete,
but it will always be 16MB.

This also means that the starting position for pg_receivexlog
is now simply right after the last complete segment, and we
never need to deal with partial segments there.

Patch by me, review by Fujii Masao

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/e7cc8437bbff99cbc7f07f852f5169ba1356a414

Modified Files
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src/bin/pg_basebackup/pg_receivexlog.c | 81 +++------------------
src/bin/pg_basebackup/receivelog.c | 123 +++++++++++++++++++++++++++++---
2 files changed, 125 insertions(+), 79 deletions(-)