Addresses one of the concerns with #7041.
Provides a mechanism (via the RPC interface) to delete a single transaction, described by its hash, from the mempool. The method returns an error if the transaction cannot be found. Once the transaction is removed it remains in the cache and cannot be resubmitted until the cache is cleared or it expires from the cache.
This changes adds an `MempoolError` field to the `ResponseCheckTx`. This will allow clients to understand that their transaction was rejected from the mempool despite passing the ABCI check.
This change also updates the code to make use of early returns to prevent highly nested code blocks. Namely, it returns when the type assertion fails at the beginning of the method, instead of wrapping the entire method in a large if statement. This has a somewhat large effect on the diff as rendered by github.
addresses: #3546
Sam Kleinman
William Banfield
Aleksandr Bezobchuk