Add a test that attempts a backup while a bulk load is happening.
Merge branch 'develop' into backup-bulk
Finish writing the test, test both the same and different sessions
so we test both the session and database handle cache behaviors.
Merge branch 'develop' into backup-bulk
Merge branch 'develop' into backup-bulk
Merge branch 'develop' into backup-bulk
Make hot backups work concurrently with files being bulk-loaded.

In summary: we don't copy hot-backup files; when we load the hot backup
metadata file during WiredTiger startup, if there are entries in the
metadata file without a valid checkpoint, we assume they're bulk-loaded
files and re-create them with no contents.

Implement Alex's idea: don't walk the metadata file in the hot-backup
code looking for 'file:' objects, instead use __wt_meta_btree_apply()
plus a callback instead.

Clarify __wt_conn_btree_apply() and __wt_meta_btree_apply(): we talk
about EBUSY errors, but all we can handle are bulk-loaded files.
Rename __wt_conn_btree_apply_single() to __wt_conn_btree_apply_bulk(),
and change it to only operate on bulk-loaded files.

Increase the size of the metadata backup line we can handle from 512B
to 5KB; this should be a dynamic value, but I'm not going to do it right
now.

Reference: #570, #653.
Merge branch 'develop' into backup-bulk-number-2
Back out the change to __wt_conn_btree_apply and __wt_meta_btree_apply:
it broke test/fops.  It's not required for hot-backups to work, remove
it entirely.
Merge pull request #674 from wiredtiger/backup-bulk-number-2

Hot backup of files that are being bulk loaded.