Turn off unit regrouping in some cases

[slogic]

There is a huge performance hit in late game when large groups of units are regrouping. Turn off regrouping when:

1. threat is far away
2. number of units in group is over some threshold, e.g. 20

[slogic]

With recent optimization of merging process MOVE command is not issued at all for groups in place of merging. So, i think there is no need for 20 unit threshold at all or raise it to 30-40.