The original design had a shared filestore write buffer and individual message blocks had a read cache.
This presented some performance and stability issues when lots of reads and deletes were happening to a
message block that was also being written to actively.
This change eliminates the shared write buffer and uses the message block's cache as a write through as
well as read cache and handles partials correctly.
Signed-off-by: Derek Collison <derek@nats.io>
fe2b354414