Check memory leaks in all tests, and fix 3 detected memory leaks

[Nao-ris]

In PR #24 I started removing memory leaks by checking them when the utility function with_db was used in unit tests.

This PR finalizes that process.

It contains a new utility class LeakChecker that checks the leaks for a unit test.
LeakChecker is added to the utility classes ReplicationTwoDbsTester and ReplicationThreeDbsTester, used to test replication use cases.
LeakChecker is also added to all tests that do not use one of the three utility function & classes.

Three new sources of memory leaks were discovered this time:

• in src/encryptable.rs, reference was used instead of take_ownership when creating a new Encryptable
• in src/replicator.rs, the struct Endpoint was not implementing Drop
• in src/replicator.rs, when a Replicator was created we were not giving ownership of the CBLDatabase object to the CBLReplicatorConfiguration object