Review Message Pool validations

[schomatis]

In contrast with incoming blocks that we try to incorporate into the chain and update its state we do lightweight validations on incoming messages (we don't project the state into the future estimating how one message might conflict with another one). As a result we are very permissive right now on what we accept increasing the pool size more than we'd ideally want. Proper validation though is expensive and we need to define exactly which trade-offs are we willing to accept.

[schomatis]

Partly addressed in #3313.