Dettached Verification + Application: ensure delay is at most one block

[igormcoelho]

Right now we following a dettached model for verification and application.
So, when transactions enter mempool, verifications are performed, and applications are only executed after block is persisted. This is important to accelerate the network and improve TPS (because block proposal and verification can happen quickly, without executing transaction applications before block is not acually final by consensus).

However, this dettached state can only last for one block difference, for two reasons:

1. if we start processing H=100, without having complete applications for H=99, we would not be sure of the GAS balances (even to pay system and network fees on next block verification), since some GAS could also be spent during application
2. if height difference gets bigger than one (example: network verification is 10 seconds, and application is 15 seconds), this means that application may never catch up to top blockchain height, and it may get delayed forever.

So, the only time we have to process applications is the time between blocks, i.e., 15 seconds.

[igormcoelho]

This solves #812 @vncoelho