Consolidate file and metafile descriptors.

Previously, a two file descriptors were used for the database: file & metafile. The "file" file
descriptor was used for async writes while the "metafile" file descriptor was used with O_SYNC
writes. This commit changes that so that there's only one file descriptor and it uses fdatasync()
to synchronize writes.
Merge pull request #109 from benbjohnson/consolidate-file-descriptors

Consolidate file and metafile descriptors.