Use dhandle hash lists in more places.  Adjust sweep timings. #1460
Extend time in test_sweep01.py. #1460
Switch to stashing the metadata dhandle, not the btree.

Also update checkpoint to use the saved handle instead of
searching for it again.
Ensure metadata table is open at start of checkpoints.
Remove redundant change from dhandle hashing.
Merge branch 'develop' into dhandle-hash
Add a few more sweep stats to understand why the test sometimes fails.
Increase test time to 120 seconds.
Update the swept handles statistic any time we close the underlying handle, regardless of whether a session still references the data handle.

reds #1460, #1466
Merge branch 'develop' into dhandle-hash

Conflicts:
	test/suite/test_sweep01.py
Merge pull request #1466 from wiredtiger/dhandle-hash

Use dhandle hash lists in more places.  Adjust sweep timings. #1460