[net processing] Only call GetTime() once in SendMessages()
[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 [[https://github.com/bitcoin/bitcoin/pull/21236 | 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

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