Add a flag to WT_SESSION::drop so that if it cannot get exclusive access to a handle, it instead flags the handle for a background drop. The namespace will not become available for reuse until the background drop is processed.

The idea is that the sweep server will look for queued drop requests and process them once the handle becomes available.

Note this requires thought if there is a crash after the drop call returns but before the drop completes. This will require logging to become crash-safe, or space may leak on disk.