feat: high-level connection manager and cancel support

[sunng87]

This patch adds advanced query cancel support. This is done by:

• Add a PidSecretKeyGenerator trait and a default implementation
• Add a ConnectionManager to hold authenticated ConnectionHandle, for cross-connection communication
• Updated default CancelHanlder implementation to use ConnectionManager to cancel query
• Updated all built-in StartupHandler implementation to accept optional ConnectionManager and custom PidSecretKeyGenerator.

All changes in this patch are optional. Users who don't use cancel feature will not be affected.