Asynchronous transactions (polling based for now). (#1652)

* Asynchronous transactions (polling based for now).

- Alter eth_sendTransaction to be async, returning one of:
  - Transaction hash (signed and submitted).
  - Transaction promise ID (< 32 bytes).
  - Zero hash (will never be signed).
- Introduce new JSONRPC eth_checkTransaction.

The new API call takes a single argument - a promise ID. It returns
either:
- Transaction hash (signed and submitted).
- null (still pending, call again later),
- Zero hash (will never be signed).

* New RPC eth_postTransaction

- Restore previous semantics of sendTransaction.
- Introduce eth_postTransaction.
- Some refactoring.

* Fix minor lockup.

* Use TransientHashMap to prevent leak.