Don't copy paths from hits staging > 2048 chars

* Silently discard paths over 2048 characters in length on import
* We have a hard limit of 2712 bytes as an index size for Postgres
  (1/3 the page buffer size)
* We have a limit of 1024 chars for hits.path in mySQL, but mySQL cheerfully
  truncates it
* Our limit for hits.path in Postgres is 2048 characters
* There are no more than 17 paths in our entire hits corpus (at time of commit)
  that exceed this, and of these, most are misdirected ASP.NET ViewState
Merge pull request #385 from alphagov/postgres-discard-long-hits-paths

Don't copy paths from hits staging > 2048 chars