Change HLL* to PF* in error messages
Fixed pfadd/pfcount commands emitting hll* events instead of pf* events
Merge pull request #1646 from raydog/unstable

Change HLL* to PF* in a few spots
HyperLogLog: added magic / version.

This will allow future changes like compressed representations.
Currently the magic is not checked for performance reasons but this may
change in the future, for example if we add new types encoded in strings
that may have the same size of HyperLogLogs.
hll-gnuplot-graph.rb improved with new filter.

The function to generate graphs is also more flexible as now includes
step and max value. The step of the samples generation function is no
longer limited to min step of 1000.