Refactor session manager

[breedx-splk]

This relates to #563 and #571. Hopefully this is a more contained, smaller, reviewable unit.

It does the following:

• renames the SessionManager implementation to SessionManagerImpl and makes it internal
• extracts a SessionManager interface from the impl
• moves the session api to its own top-level module
• moves SessionidTimeoutHandler from java to kotlin and makes it internal

The intended next step is to add SessionManager to the InstallationContext and create a new instrumentation module that can generate session start/end events, as in #571.