Remove ResponseCache in favor of MDMFSlotReadProxy's cache. closes #1240.

Remove ResponseCache in favor of MDMFSlotReadProxy's cache. closes #1240.

This contains several merged patches. Individual messages follow, latest first:

* Fix a warning from check-miscaptures.
* In retrieve.py, explicitly test whether a key is in self.servermap.proxies
  rather than catching KeyError.
* Added a new comment to the MDMF version of the test I removed, explaining
  the removal of the SDMF version.
* Removed test_corrupt_all_block_hash_tree_late, since the entire block_hash_tree
  is cached in the servermap for an SDMF file.
* Fixed several tests that require files larger than the servermap cache.
* Remove unused test_response_cache_memory_leak().
* Exercise the cache.
* Test infrastructure for counting cache misses on MDMF files.
* Removed the ResponseCache. Instead, the MDMFSlotReadProxy initialized
  by ServerMap is kept around so Retrieve can access it. The ReadProxy
  has a cache of the first 1000 bytes initially read from each share by
  the ServerMap. We're able to satisfy a number of requests out of this
  cache, so roundtrips are reduced from 84 to 60 in test_deepcheck_mdmf.
  There is still some mystery about under what conditions the cache has
  fewer than 1000 bytes. Also this breaks some existing unit tests that
  depend on the inner behavior of ResponseCache.
* The servermap.proxies (a cache of SlotReadProxies) is now keyed
  by (verinfo,serverid,shnum) rather than just (serverid,shnum)
* Minor cosmetic changes
* Added a test failure if the number of cache misses is too high.

Author: Andrew Miller <amiller@dappervision.com>
Signed-off-by: David-Sarah Hopwood <davidsarah@jacaranda.org>