query_supervisor: filtrating limited users

query_supervisor needs to be installed in postgres pkglib folder
the only safe method of configuration is to load it with every session
is SET  pg option 'session_preload_libraries' TO 'libquery_supervisor.so'
query_supervisor add configuration option 'query_supervisor.limited_users'
with list of user names separated with a comma. If user which opens
session is in the list, then his queries are limited.