One pass walks the handle list without locking and finds candidate handles to close, and the second pass holding the handle list lock that removes dead handles.
refs WT-1814, WT-1811, WT-1808
WT-1811 Change sweep to not wait on the dhandle list lock
WT-1808 Avoid a deadlock between the sweep server and a checkpoint.
WT-1814 Deadlock caused by sweep server