Remove allocations from read-only buckets.
Avoid trashing page cache on Tx.Copy().

This commit change the database copy to use O_DIRECT so that the Linux page
cache is not trashed during a backup. This is only available on Linux.
Merge pull request #172 from benbjohnson/allocation

Remove allocations from read-only buckets.
Merge branch 'master' of https://github.com/boltdb/bolt