Smarter transaction scheduler

[kostko]

Runtimes can provide metadata to the scheduler via the Meta field in the CheckTx response.

It would be useful to standardize a few metadata items that the runtimes can emit and the scheduler understands while scheduling batches:

• Priority.
• Custom "weights" where for each "weight type" the runtime can define a maximum block weight. This could be gas or emitted runtime messages. The maximums could either be emitted by the runtime as part of the last ExecuteTx response or queried explicitly by the host via a new call (per round).