Add sufficient tests for v0 and fix issues with SendAndHold.

Implement access to mempool state and refactor transactor to use this to
set sequence number optimistically.
Swap order of check cache reset and committer commit to avoid
concurrency issues with mempool access during commits.
Make transactor accept InputAccount for all signing methods and add
InputAddress parameter to V0 methods allowing address based signing in
addition to private key based signing with minimal changes.

Signed-off-by: Silas Davis <silas@monax.io>