Improved cleanup after session disconnection

Ignore-this: 7f24d23107dc2d98ad7c03aa2a43b56c

darcs-hash:20100420113356-1e3d8-6cd36e53876803706d1af6794edc944e5bf19bc7
Added fix for receiving a BYE without receiving a final response to INVITE

Ignore-this: 8083f9b1185b3791ddba07571bac27b4

darcs-hash:20100420121418-1e3d8-6dbf4a208555869293714aa5963ccc67bea3c745