Made the dbm_read_ahead_fsm_t take a buf_out parameter that points at the true (serializer viewpoint) beginning of the buffer.
Moved data_block_manager_t::read assertions about ser_block_size_in and off_in inside the part of the function that actually has that antiquated requirement.
Made data_block_manager_t::read no longer have ls_buf_data_t subtraction logic -- that is pushed out to the log_serializer_t.