session support

[MichaelScofield]

What problem does the new feature solve?

GreptimeDB lacks the session layer, which then makes tracking client's status/quotas impossible.

What does the feature do?

Adds a session layer to our db, so as to store client's status and quotas.

Implementation challenges

To start, stores the database in "USE" stmt in session. see #670

[MichaelScofield]

We now have some context mods like QueryContext or UserInfo in session crate. This issue is too broad to track anything meaningful. We should create new issues for more specific demand.