[net processing] Only call GetTime() once in SendMessages()
Summary:
We currently call GetTime() 4 times in SendMessages(). Consolidate this to once GetTime() call.
This is a backport of core#21236 [1/6]
https://github.com/bitcoin/bitcoin/pull/21236/commits/c02fa47baa517e17b5c43bde3902b1e410c1b93f
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D10915