Add a reference count to tables, only free if the table is not in use.
In drop, don't release a table we didn't get.
If a create fails, remove the table from the session.
Deal with errors during wiredtiger_open if a shared cache is configured.
Merge pull request #466 from wiredtiger/table-refcount

Add a reference count to tables, only free if the table is not in use.
Don't pound on disks by default in test/format -- we use it for lots of automated tests and direct I/O plus checkpoints makes it *much* slower than otherwise.  They can always be turned on with WIREDTIGER_CONFIG.