Add the ability to touch a key's write time

[whitfin]

This is a prerequisite to having good LRU.

Slightly different to refresh in that we need to calculate any potential TTL deltas to make sure that it still expires at the same time.

For example:

# assume now is 1000ms

set({ "key", 1000, 500, "value" }) # would expire in 500ms

:timer.sleep(100)

touch("key") # this should become { "key", 1100, 400, "value" } so it still expires in 400ms