Sessions now track their creation time and are automatically cleaned up after SESSION_TIMEOUT (30s) when a new session is opened. This prevents stale sessions from accumulating when clients crash or disconnect without calling close(), which would otherwise permanently exhaust the 16-session limit and make the CDM unusable until server restart.
Mirrors the existing implementation in csplayready.
Closes#4
+ Updated RemoteCdm/Serve to support RevLists
+ Added Storage class for RevList persistence
+ Added support for ExtData objects in BCerts + signature verification
+ Added an Xml Builder class (instead of xmltodict)
+ Added a SOAP Builder/Parser class (instead of xmltodict)
+ Refactored WRMHeader class to use ET (instead of xmltodict)
+ Upgraded ServerException detection
+ Removed dependencies: xmltodict, lxml
+ Added Util class
+ Minor Crypto/ElGamal class changes
