Improve const-ness of server classes.

[RobertBlakeAnderson]

Rebased fork to re-attempt #73

[RobertBlakeAnderson]

Looks like the checks are failing due to an issue with boost:
no matching function for call to ‘boost::unique_lock<boost::recursive_mutex>::unique_lock(const boost::recursive_mutex&)’ boost::recursive_mutex::scoped_lock lock(this->lock_);

Not sure what the issue is here.

[progtologist]

You cannot lock a mutex in a const function. You should change the recursive mutex to mutable to fix the compilation issue.