pt-stalk magical algorithm

Create a magical trigger function

The magical trigger function will watch SHOW STATUS and track these variables:

   a. Queries
   b. Threads_running
   c. Threads_connected

Queries must be tracked between samples, so it subtracts the last sample from
the current sample, divides by elapsed time, and obtains queries per second.

Each of these should be turned into a long-term exponentially decaying average,
by keeping an accumulator variable. When all three of them cross a threshold,
the tool triggers. The exact definition of the threshold is to be specified
later after more research.