This change has two main effects:
1. Remove most of the Async methods from the abci.Client interface.
Remaining are FlushAsync, CommitTxAsync, and DeliverTxAsync.
2. Rename the synchronous methods to remove the "Sync" suffix.
The rest of the change is updating the implementations, subsets, and mocks of
the interface, along with the call sites that point to them.
* Fix stringly-typed mock stubs.
* Rename helper method.