Improved debugging

• examples: wrap task.destroy with an exception handler
• add GC stats reporting
• support a "feature-suspend" flag
• fix HTTP requests with an empty cookie
• add opam file
• add documentation
• update to new backtrace API
• xe: add "--trace" option to print backtraces
• add license check for vGPU
• run migration pre-checks for VDI.pool_migrate
• tidy up the internal database interface