MySQL config backend must not set timers to default value of 0

Currently, when the MySQL Config Backend finds NULL DHCP timer value (e.g. renew-timer) it will use a value of 0 as a default. The subnet logic, though, treats unspecified value differently than the 0 value. The 0 value has a meaning (at least in v6) that it is up to the client to determine when to renew. The unspecified value causes the server to calculate the times based on the percentages.

We need to adjust the backend to store unspecified timers (triplets) as NULL. Then, the backend has to turn NULLs to unspecified triplets.