Clear the message id of the user.

Clear the message list of the user.

Create root message and return the root message id.

Get the conversation info by conversation id.

Get the message list of the user.

Determine whether the message is the root message.

Link a message to other message as child.

Set the message as roote message.

storage the links between parent message and child message.

storage the messages.

storage the user's conversations.

storage the user's messages.