[enhancement] add a session manage module

[xuanyuan300]

Feature Request

Is your feature request related to a problem? Please describe:

Describe the feature you'd like:

After added the readwritesplitting feature, the processing of ConnAttr has been added. Currently, the attrs processed are charset, autocommit, but the processing logic is integrated in Pool, and Pool is a public component, different databases may also there are different session attrs, so ConnAttr should be separated from Pool.

Describe alternatives you've considered:

Add a specifiy session manage module.

Teachability, Documentation, Adoption, Migration Strategy:

[xuanyuan300]

WIP, this issue will reslove in v0.3.0.

[mlycore]

It is useful for backend database endpoint rolling upgrade without any discruptions of applications.

[xuanyuan300]

In pr #274, Added Session trait and SessionMut trait.