[net] Thread safety annotations in net_processing

Summary:
Annotate AssertLockHeld() with ASSERT_CAPABILITY() for thread safety analysis

[net_processing] Add thread safety annotations

This is a backport of Core PR13423

Depends on D1915

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1922