rust: Mirror TROVE-2018-005 fix in Rust protover implementation.

 * REFACTORS `UnvalidatedProtoEntry::from_str` to place the bulk of the
   splitting/parsing logic in to a new
   `UnvalidatedProtoEntry::parse_protocol_and_version_str()` method (so that
   both `from_str()` and `from_str_any_len()` can call it.)
 * ADD a new `UnvalidatedProtoEntry::from_str_any_len()` method in order to
   maintain compatibility with consensus methods older than 29.
 * ADD a limit on the number of characters in a protocol name.
 * FIXES part of #25517: https://bugs.torproject.org/25517