Make sure compact blocks work when there are more than 65536 transactions in a block

Summary:
As per title. The compact block code rely heavily on the use of 16 bits integers, which made it unsuitable for the propagation of block containing a large number of transactions.

Depends on D1144

Test Plan: abc-p2p-compactblocks was updated to ensure that blocks with a very large number of transactions are passed back and forth.

Reviewers: #bitcoin_abc, dagurval

Reviewed By: #bitcoin_abc, dagurval

Subscribers: dagurval, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1146