On receiving timestamps, clock now tries to forward its delta.
It turns out that ready messages are only sent at the end of
onCreates.  Therefore, piggybacking the inital clock timestamp on
them is a bad idea (many endpoints do not have onCreate-s).  Sending
timestamp as separate message.
Had been incorrectly updating delta; not including previous delta val.
Added a test that ensures that endpoints advance their clock
timestamps and forward them to each other.
Forgot to remove access to timestamps in partner ready message generation.
Previous commits processed clock updates on connection object
receiving thread.  This could slow down receiving and processing
additional messages.  Separated sending clock update messages to all
partners (when update own clock delta) into a separate worker thread.
Filed an issue (#66) for what was warning about.  In short:

Whenever rollback in PartnerEventParent or EndpointCallEventParent,
forward the rollback to partner or endpoint that called us,
respectively.  However, if it was the partner or endpoint that called
us that initiated the rollback, we can skip sending back this message.