Cache Server should not allow file overwrites during reads

[stephen-palmer]

There is a test that presumes to allow the cache server to overwrite a file version, while that version is being sent to a client. The test was passing on both cache modules (RAM and FS), but the RAM module has a logic error that causes a false positive for this test. The test passes on the FS module due to the nature of disk caching and read streams in node that are non-locking.

Really, we shouldn't allow this behavior at all - if a file is being sent to one client, it cannot be overwritten by a different client at the same time (i.e., a PUT transaction).