Introduce ITransactionCallback, replacement for INodeSessionCallback

[smdn]

Description

This PR introduces a new interface, ITransactionCallback.
The main motives for introducing this interface are the following four.

• Provide a simpler approach than INodeSessionCallback
• Decoupling of NodeBase and transaction callbacks
• Discontinue the Session ID feature in NodeBase
• Replace "session" with "transaction," a term used in Munin documents

This interface provides a mechanism for calling back at the start and end of a request by munin-update.
Unlike the existing INodeSessionCallback, it can be implemented in any type together with IPluginProvider or IPlugin.
In addition, callbacks are changed to be triggered from IMuninProtocolHandler rather than directly from NodeBase.

INodeSessionCallback is now obsolete and will be deprecated in the next major release.
Due to this, the following features will also be deprecated.

• Session ID functionality in NodeBase
• SessionCallback property provided by IPluginProvider and IPlugin