Read caching implementation

Writeboost implements read-caching using write-caching. The basic idea is:

1. Only caches 4KB (full-sized) read requests
2. Hold read data into the read-cache cells in the endio callback (read_cache_cell_copy_data). There are 2048 cells preallocated by default. reserve_read_cache_cell is called when the read request result in cache miss, will be read from the backing store, and the function reserves a new cell. In endio, the read data payload is copied to the cell.