Refax: added size cache to the group container

[ethouris]

This adds a size cache to the group container.

The group container is std::list and as such it doesn't have its own saved cache. There were two reasons to add this:

1. In some future implementations the call to the container's size() can be even very often, while std::list original size() is linear time in the number of members.
2. There will be also situations when the size is needed to be taken without locking the group's mutex. With this cache the size can be directly taken from the cache, and as atomic, it doesn't need to be mutex-locked.