Merge #13907: Introduce a maximum size for locators.

e254ff5d53b79bee29203b965fca572f218bff54 Introduce a maximum size for locators. (Gregory Maxwell)

Pull request description:

  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.

  Locators are cheap to process so allowing a few more is harmless,
   so this sets the maximum to 64-- which is enough for blockchains
   with 2^64 blocks before the get overhead starts increasing.

Tree-SHA512: da28df9c46c988980da861046c62e6e7f93d0eaab3083d32e408d1062f45c00316d5e1754127e808c1feb424fa8e00e5a91aea2cc3b80326b71c148696f7cdb3

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