* Add txmgr rpc api

* Update mock TxManager

* Use parameterized tests to remove redundant code

* Add txmgr.cfgLock to protect values configurable at runtime

* txmgr: use generic API() method on interface to allow custom rpc apis

* txmgr: re-generate mocks

* txmgr: use atomics for Config vals that can be modified via rpc

* txmgr: use pointer for SimpleTxManager.cfg

* txmgr: remove extraneous code

* txmgr: cleanup ctx input arg in rpc methods