add memory pool capacity check and transaction expiration check

[Jim8y]

#2862

[vncoelho]

@Liaojinghui,

In this line of thinking, a good direction for neo-core code could be:

• Create a first PR that is focused on a design for NEO C# Core to handle "alternate mempool"
• The pool can be "next-block-or-never inclusion" (such as the one you proposed here)
• The pool can be any other strategy with fees, among others
• These pools can also be added as plugins
• CNs could handle a list of mempool and sort them first, before picking txs. Thus, there could be a parameter that sets priorities of pools (committee role)

I think it is worth checking Ethereum EIP 4337 which also has "alternate mempool" concepts for a nice feature focused on account abstraction.

[Jim8y]

@vncoelho Thank you Vitor for informing me that EIP, will learn and try to optimize this pr.