Introduce a maximum size for locators.
The largest sensible size for a locator is log in the number of blocks. But, as noted by Coinr8d on BCT a maximum size message could encode a hundred thousand locators. If height were used to limit the messages that could open new attacks where peers on long low diff forks would get disconnected and end up stuck. Ideally, nodes first first learn to limit the size of locators they send before limiting what would be processed, but common implementations back off with an exponent of 2 and have an implicit limit of 2^32 blocks, so they already cannot produce locators over some size. This sets the limit to an absurdly high amount of 101 in order to maximize compatibility with existing software.
Backport of core PR13907 and PR13915:
ninja check test_runner.py p2p_invalid_locator
Reviewers: #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D4469