Use jemalloc as a default

Summary:
Including the gitian builds. Minimum version is set to 3.6.0 which is
widely distributed (tested on Ubuntu 18.04).

Depends on D6365.

Test Plan:
Read the docs.
Run the gitian build twice on different machines and check the build is
deterministic.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6042