Fix incorrect tps number calculation in "excluding connections establishing".

The tolerance (larger than actual tps number) increases as the number
of threads decreases.  The bug has been there since the thread support
was introduced in 9.0. Because back patching introduces incompatible
behavior changes regarding the tps number, the fix is committed to
master and 9.5 stable branches only.

Problem spotted by me and fix proposed by Fabien COELHO. Note that his
original patch included more than fixes (a code re-factoring) which is
not related to the problem and I omitted the part.