Add mutex lock to Actions.update.
Add mutex lock to Actions.update.

The apply event expects the mutex to be locked when it is called via
ledger.Add. update was fine before because it was only being called from
the worker task handlers, which were only called under Actions.do, which
previously held the lock for the whole function. Now that it releases
the lock before sending a task to the worker, any call to update needs
to get the lock again.

dsrbecky/gapid,pmuetschard/gapid,google/gapid,google/gapid,baldwinn860/gapid,Qining/gapid,ek9852/gapid,pmuetschard/gapid,google/gapid,google/agi,ek9852/gapid,ek9852/gapid,baldwinn860/gapid,ek9852/gapid,dsrbecky/gapid,baldwinn860/gapid,google/gapid,google/agi,pmuetschard/gapid,google/agi,google/gapid,Qining/gapid,Qining/gapid,baldwinn860/gapid,ek9852/gapid,google/gapid,ek9852/gapid,dsrbecky/gapid,dsrbecky/gapid,pmuetschard/gapid,ek9852/gapid,google/gapid,google/gapid,dsrbecky/gapid,baldwinn860/gapid,baldwinn860/gapid,dsrbecky/gapid,ek9852/gapid,baldwinn860/gapid,Qining/gapid,pmuetschard/gapid,Qining/gapid,dsrbecky/gapid,pmuetschard/gapid,google/agi,Qining/gapid,pmuetschard/gapid,google/agi,dsrbecky/gapid,Qining/gapid,Qining/gapid,google/agi,baldwinn860/gapid,pmuetschard/gapid,google/agi,google/agi