Cache Improvement

[kumavis]

• breakout into separate file
• handle in-flight caching ( subsequent requests while actively handling request )
• add cache "roll-off" with option for number of blocks before rolloff
• handle fork-level caching ( requires fork-detection Block Polling #4 )

[tcoulter]

In flight caching already exists since the pipe can only handle a single request at a time, right?

[kumavis]

incorrect - rpcs can happen simultaneously, and you can have 10 requests for the same balance at the same time, and all will miss the fact that others are in flight

[kumavis]

split into #29 and #28