Take locks while dumping the cache to avoid crashes

While dumping the cache during a cache stuck clean situation, the system took an assertion failure. The console output of the failing job is: http://build.wiredtiger.com:8080/job/wiredtiger-test-format-stress/65894/console.

The job is identified by Jenkins as a cache stuck clean but ending up at an assertion failure is new.

0x7fcef9d4e1d0:cache clean check: yes (95.136%)

0x7fcef9d4e1d0:cache dirty check: no (12.510%)

[1524038874:891184][36664:0x7fcef7b49700], t, file:wt, eviction-server: __tree_walk_internal, 312: btree->root.page != NULL

[1524038874:891189][36664:0x7fcef7b49700], t, file:wt, eviction-server: aborting WiredTiger library

Dumping the cache does not acquire locks so maybe this is just a rare situation where that matters. The initial failure was the full cache.