Speed up execution when pinba.enabled is set to 0

[gggeek]

Atm, when pinba.enabled is set to 0, the extension code does still create timers, even though it does not flush them later on. The process of doing that does consume both cpu time (at least a gettimeofday call), and memory.

In order to minimize the impact of having one's code permanently sprinkled with pinba timer calls, it would be nice if such resource consumption was as limited as possible when pinba.enabled is set to 0.

Given that we now have the pinba.autoflush setting available to let users set up the code in a way that statistics are collected but not sent, it stands to reason that the BC break of changing the extension behaviour so that timers and tags are not created at all when pinba.enabled is set to 0 might be perceived as worthwhile