Compaction: Don't write to /tmp

As part of investigating MB-11557, I discovered that couch_compact actually writes to files outside the data directory. Specifically it creates 3 temporary files (using tmpfile() ), which from an initial glance appear to be used to build the new, compacted b-tree (please correct me if I'm wrong).

This is *bad* for a number of reasons:

1) Nowhere in our documentation or install guide do we mention that /tmp is needed by Couchbase.
2) If /tmp isn't writable, then compaction fails (with an opaque error message) - see MB-11557. Note nothing else seems to require /tmp, so the system works perfectly apart from your disks slowly running out of space.
3) Related to (1), while users typically configure a fast IO subsystem for the data and index directories they have no reason to do so for /tmp - and so this may be on a slow subsystem, creating a bottleneck for IO

Ideally we wouldn't use additional temporary disk space at all (and just use an mmap()ed file / memory), but we should at least write to the same directory as the output file, which is already in the data / index directory and can be assumed to be fast.

Additional info:

Attached strace file shows the writes/reads made to the three temporary files while compacting an example couchdb file:

/opt/couchbase/bin/couch_dbinfo 9.couch.2
DB Info (9.couch.2)
file format version: 11
update_seq: 35961
doc count: 19946
deleted doc count: 0
data size: 2.59 MB
B-tree size: 1.11 MB
total disk size: 10.18 MB

Approximately 3.2MB was written to temporary files (rough grep of writes to FDs 5 and 6).

Output (compacted) file:

[vagrant@localhost vagrant]$ /opt/couchbase/bin/couch_dbinfo couch.compacted
DB Info (couch.compacted)
file format version: 11
update_seq: 35961
doc count: 19946
deleted doc count: 0
data size: 2.64 MB
B-tree size: 1.16 MB
total disk size: 2.64 MB