[net] Add RunInactivityChecks()

Summary:

Moves the logic to prevent running inactivity checks until the peer has been connected for -peertimeout time into its own function. This will be reused by net_processing later.

Partial backport of core#20721:
https://github.com/bitcoin/bitcoin/pull/20721/commits/1a07600b4b0d08cffc7cd5c58af33fcd1ede558e

Depends on D10873.

Ref T1696.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D10875