Switch from using the checkpoint lock to serialize bulk-load close and
database checkpoints, to using a new WT_BTREE handle lock.
Reference #1114.
Merge branch 'develop' into bulk-checkpoint

Conflicts:
	src/txn/txn_ckpt.c
Revert "Switch from using the checkpoint lock to serialize bulk-load close and"

This reverts commit 51de923778091761c99a48f5e786c7cef39f73da.
Switch from using the checkpoint lock to serialize bulk-load close and
database checkpoints, to using a new WT_DATA_HANDLE lock.  Reference
Merge pull request #1126 from wiredtiger/bulk-checkpoint

Replace checkpoint lock with btree handle lock to serialize with bulk load.