Conditionally release memcached clients

This pull request replaces `EmceeStore.releaseClient()` with `EmceeStore.releaseWithout()`, which ensures that bad connections will not be returned to the memcached connection pool. The method name is a mnemonic for its usage—`releaseWithout(client, &err)` == "release without client error"—but I can rename it if that doesn't make sense.

@bbangert r?