SERVER-13922 Remove Context and dbtemprelease from the cloner

Substituting with plain TempRelease, combined with re-acquiring the
database after lock is re-acquired.

Since copydb/clonedb are not shard-aware operations, we skip the shard
version check.

The usages, which originate from replication are all from the secondaries,
so there is no need for version check there either.
SERVER-14178 Do NULL check before dereferencing