net: Move SocketSendData lock annotation to header

Summary:

Also, add lock annotation to SendMessages

Can be reviewed with "--word-diff-regex=."

Completes backport of core#20864:
https://github.com/bitcoin/bitcoin/pull/20864/commits/fa210689e27b0d78b2fe894e51e364179db7a3ce

Depends on D10880.

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/D10881