prevent multiple threads from sending/receiving messages to the same child/session concurrently
prevent multiple threads from sending/receiving messages to the same child/session concurrently

git-svn-id: ec97508af0aa29a1d296967d6f0ba22a468c79d6@271 286bb87c-ec97-11de-a004-2f18c49ebcc3

rctk/rctk,rctk/rctk