Ensure the http server accept request at least twice as large as the block size.

Summary:
As per title. This ensures it is always possible to submit blocks via RPC.

Fixes T319

Test Plan: Added a test case to abc-p2p-fullblocktest where a very large block is submited via RPC.

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: schancel, teamcity

Maniphest Tasks: T319

Differential Revision: https://reviews.bitcoinabc.org/D1285