sampling: do not coerce intervals in two classes

previosu code cut corners and had just two
classes of intevals: "fast" (<= 15s interval)
and "slow" (everything else)

this patch lift this limit and let the sampling
have any number of intervals.

In worst case scenario, each sampler will have
a different interval, thus a different pool.
We cannot avoid that, just hope there is a good
reason for this.
thpool: make Watchman stoppable

We need to be nice and clean up things once done
thpool: make the pool stoppable

We need to be nice and clean up things once done
thpool: try to dispose the terminated rogues

explicitely handle terminated rogues
thpool: update the TODO
thpool: add docstrings and pylint fixes.
thpool: submit work only if the pool is started

this hunk was mistakenly took away in a former commit.