Add sector caching/pooling

Right now, PSXACT loads each and every sector on request. This could be more efficient if instead of loading sectors one by one, the emulator loaded sectors from a cache. The cache could be as simple as

If a cache miss occurs, flush and load the requested sector, and all of the sectors with the same mm:ss:** timecode.

Very often, the sectors being read are data sectors, and are being requested by the BIOS fread function. In this case, sectors are commonly read sequentially and the sequential cache would reduce the overhead of this considerably.