Bogus cache_block behaviour after blockBytestream changes

• AStreamSeekBlock(i_pos) no longer allows backward seek (seek in cache) as it caches only forward
• AStreamSeekBlock(i_pos) is also an incorrect read size since this is the global offset
• No longer uses differential logic for seeking / evaluates cost of underlying seek
• On max cache reached, flushes the whole cache instead of just making room. Goes unnoticed because backward caching is broken

All changes by 9a725a03