Transaction scheduler should use CheckTx

[kostko]

As of #1555 the runtime exposes a CheckTx API for verifying incoming transactions but this is currently unused. The transaction scheduler should use the API to reject incoming transactions early instead of submitting them through the processing pipeline.

The runtime must ensure that any such checks are cheap enough that they can be performed quickly and not become a bottleneck of the whole pipeline.

Estimated cost: 1 sprint

[peterjgilbert]

The runtime must ensure that any such checks are cheap enough that they can be performed quickly and not become a bottleneck of the whole pipeline.

Currently the runtime checks are cheap (https://github.com/oasislabs/runtime-ethereum/blob/9847c5359d04a6ea423739f70f2f511ea37dbf66/src/methods.rs#L27):

1. Decode the transaction
2. Check the signature
3. Check that gas limit and gas price have allowed values

[kostko]

The checks also need to make sure that there is enough balance to pay for gas in case the transaction is reverted due to a mismatched predicted read/write set (see #2012 (comment)).

[kostko]

As discussed the CheckTx API should be extended so that the return value includes additional metadata (e.g., predicted read/write sets for the transaction). The existing way of transporting read/write sets should be cleaned up.

[kostko]

Performing CheckTx should be a node configuration parameter so it can be disabled if needed.