[MEDIUM] stick_table: separate storage and update of session entries

[MEDIUM] stick_table: separate storage and update of session entries

When an entry already exists, we just need to update its expiration
timer. Let's have a dedicated function for that instead of spreading
open code everywhere.

This change also ensures that an update of an existing sticky session
really leads to an update of its expiration timer, which was apparently
not the case till now. This point needs to be checked in 1.4.