Add wal upload concurrency

This allows significantly higher write throughput and, in doing so,
can help avoid WAL archiving backlogs while undergoing heavy write
workloads.

The general approach is to upload multiple WAL segments with one
command invocation, by directly manipulating the
"pg_xlog/archive_status" directory.

Mechanically, this means identifying a SEGNAME.ready file, uploading
the corresponding segment, and then renaming the .ready file to
SEGNAME.done should the upload complete successfully.  All versions of
PostgreSQL so far make simple use of the archive_status directory to
identify the next file to upload, and raise no concern with extra
segments being marked as complete (.done) outside the archive_command
contract.

This is not a documented behavior of or contract with PostgreSQL, but
these internal structures haven't changed much since inception and the
gains are too large to ignore.

This functionality is left off (but made available) in the v0.6 series
by default to avoid changing behavior for those upgrading from an
older point release.