Skip unnecessary calls to SecTrustEvaluate

I learned that the trust has usually already been evaluated by the caller, so
calling SecTrustGetTrustResult is usually enough.
Avoid 409 Conflict errors caused by duplicate replicators

Prevent two CBL_Replicator instances with the same settings from being
active at the same time. Also ensure that replicators with different settings
always have different checkpoint doc IDs to avoid collisions.
Fixes #92