Adding lock in TransportManager::flushSession().
Adding lock in TransportManager::flushSession().

This allows servers (i.e., multithreaded processes) to use it
safely.

anirajk/RAMCloud,jblomer/ramcloud,SMatsushi/RAMCloud,alexandermerritt/ramcloud,alexandermerritt/ramcloud,jblomer/ramcloud,matrix207/RAMCloud,anirajk/RAMCloud,alexandermerritt/ramcloud,QingkaiLu/RAMCloud,QingkaiLu/RAMCloud,utah-scs/RAMCloud,rstutsman/RAMCloud,rstutsman/RAMCloud,rstutsman/RAMCloud,rstutsman/RAMCloud,matrix207/RAMCloud,QingkaiLu/RAMCloud,jblomer/ramcloud,matrix207/RAMCloud,utah-scs/RAMCloud,anirajk/RAMCloud,QingkaiLu/RAMCloud,jblomer/ramcloud,alexandermerritt/ramcloud,anirajk/RAMCloud,SMatsushi/RAMCloud,jblomer/ramcloud,alexandermerritt/ramcloud,jblomer/ramcloud,matrix207/RAMCloud,utah-scs/RAMCloud,rstutsman/RAMCloud,matrix207/RAMCloud,SMatsushi/RAMCloud,QingkaiLu/RAMCloud,anirajk/RAMCloud,alexandermerritt/ramcloud,anirajk/RAMCloud,QingkaiLu/RAMCloud,SMatsushi/RAMCloud,SMatsushi/RAMCloud,utah-scs/RAMCloud,rstutsman/RAMCloud,utah-scs/RAMCloud,SMatsushi/RAMCloud,matrix207/RAMCloud,utah-scs/RAMCloud