DRILL-5485: Remove WebServer dependency on DrillClient
1. Added WebUserConnection/AnonWebUserConnection and their providers for Authenticated and Anonymous web users.
2. Updated to store the UserSession, BufferAllocator and other session states inside the HttpSession of Jetty instead
of storing in DrillUserPrincipal. For each request now a new instance of WebUserConnection will be created. However
for authenticated users the UserSession and other states will be re-used whereas for Anonymous Users it will created
for each request and later re-cycled after query execution.
close #829
