add ability to enable/disable quantiles for Timers/Histograms

[donbourne]

This should include the ability to enable/disable by configuration
This may also include the ability to enable/disable programmatically

[Channyboy]

Related to #675
There will be no explicit MP configuration for enabling and disabling quantiles.
Quantiles will be enabled by default.

Quantiles can be disabled through configuration of the quantile values by setting a value.

i.e.

mp.metrics.distribution.percentiles=	                            // means no percentiles
mp.metrics.distribution.percentiles=*=	                            // means no percentiles for all metrics
mp.metrics.distribution.percentiles=*=;a=0.5                    // means all off, except metric a 
mp.metrics.distribution.percentiles=some.metric=;a=0.5  // means some.metric disabled quantiles