Update p2p reqresp with explicit handling of reqresp size bounds

[mbaxter]

Description

Apply changes from spec PR: ethereum/consensus-specs#1800

The v0.12.1 networking spec states:

For both requests and responses, the encoding-dependent-header MUST be valid, and the encoded-payload must be valid within the constraints of the encoding-dependent-header. This includes type-specific bounds on payload size for some encoding strategies. Regardless of these type specific bounds, a global maximum uncompressed byte size of MAX_CHUNK_SIZE MUST be applied to all method response chunks.

Clients MUST ensure that lengths are within these bounds; if not, they SHOULD reset the stream immediately. Clients tracking peer reputation MAY decrement the score of the misbehaving peer under this circumstance.

See "What are SSZ type size bounds?" for an explanation of how to calculate type-specific chunk size limits.