Revert "Made store_view_t<>::write take a const signal_t."

This reverts commit 987632232722bc0f8873f99a0fefd4dbc56b33ba.
Passed disk_ack_signal down to btree_store, but no further.
Passed disk_ack_signal down to protocol_write.
Passed disk_ack_signal to write_visitor_t.
Revert "Passed disk_ack_signal to write_visitor_t."

This reverts commit f1e1040516797d480ea0565fd6d4b225b225de3f.
Revert "Passed disk_ack_signal down to protocol_write."

This reverts commit b97ff9a95c0e580bcaeda23ba52dc740067b3341.
Pushed the disk_ack_signal to the transaction_t constructor.
Removed the mock cache.