Summary:
It is useful to have per-session shared memory between tserver and pggate (this is needed, for example, by D43907 to share a lock owner tag). We currently already have per-session shared memory files today, but it is managed and used solely by `SharedExchange`.

This diff extracts the shared memory file management portion of `SharedExchange` into a new class `PgSessionSharedMemoryManager`, and `SharedExchange` has been changed to only operate on a portion of the shared memory file (`SharedExchangeHeader` is currently the only field of `PgSessionSharedHeader`, so `SharedExchange` still operates over the entire shared memory file, but future changes will add additional fields unrelated to `SharedExchange`).
Jira: DB-17026

Test Plan: Jenkins

Reviewers: sergei, dmitry

Reviewed By: sergei

Subscribers: yql, ybase

Differential Revision: https://phorge.dev.yugabyte.com/D44417