Mutable State Version & Activity Heartbeat Timer

[yycptt]

Currently the mutable state is "versioned" using next event ID; next event ID is not a good attr for protecting concurrent modification of mutable state or preventing writing the activity heartbeat > 1 times.

We should have a additional attr 'dbVersion', which will +1 for each db write related to a mutable state

We can also consider persist the heartbeat timer time along with activity (currently it is a in mem only attr)

[yycptt]

For background context, check: #3032

[mkolodezny]

opened issue in temporal temporalio/temporal#435 to track