Allow large integers to be used in maxtries

Summary:
While doing some testing, I ran into this issue.
maxtries is the total number of tries regardless of the number of blocks to generate.
On modern machines, it's trivial to hit the limit when generating hundreds of blocks in
one go. The underlying implementation already supports 64-bit int, so let's use it.

Test Plan:

ninja
test_runner.py mining_basic

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7138