Add computation limits on transactions

[cmwaters]

Summary

We want to prevent situations that could lead to excessive computation which could also slow down the network (This could happen if all the transactions in an 8MB block were IBC headers for example).

Proposal

We could introduce an upper bound on the amount of txs or we could use the MaxGas field in the ConsensusParams.

[cmwaters]

One of the first steps would be to benchmark the effects of "computationally heavy" transactions on execution by seeing how long it would take to execute 1000 IBC headers or 5000 Send Txs