Optimization: Avoid serialization and deserialization of session data by default

As part of changes for Stash Data Center, sessions now go through serialization and deserialization even in standalone Stash instances where it is not required. Normally this is relatively inexpensive, but if sessions contain a large amount of data (for example, if custom plugins store additional information in the session), the costs can add up.

We should optimize this so that session data avoids serialization and deserialization by default.