Scale disconnected transaction pool size based on maximum block size

Summary: T168: MAX_DISCONNECTED_TX_POOL_SIZE was added by a Core PR, but doesn't consider block size when defining this value. I've added a scaling factor and changed it from kilobytes to bytes to be consistent with blocksize.

Test Plan: make check

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1009