Possibility to set log level for a remote session

[svinniks]

It must be possible to set log level for any session, not only the current one.
There must be an (optional) argument for SET_SESSION_LOG_LEVEL, which takes session's serial# (because it's unique). If NULL has been passed, current session serial # must be used.
Globally accessed context must be used to store/check session levels.
To avoid high memory usage finished session levels must be cleaned up time after time (for example, each 100-th call to SET_SESSION_LOG_LEVEL).
Functionality must be checked for proxy sessions!.