[ReadHandler] Unify Now timestamps

[lpbeliveau-silabs]

In the report Scheduler, we are calling get monotonic timestamp several times in a row to establish a "now" moment. Rework needed so the same timestamp is used instead of consecutive calls to get monotonic timestamp.

[bzbarsky-apple]

The reason this matters: we can get radically different (hundreds of milliseconds apart) values for two consecutive now() calls on Mac/Linux, and the code very much does not expect that to happen as it's written now.

[lpbeliveau-silabs]

See here:
https://github.com/project-chip/connectedhomeip/pull/27553/files#r1260284902
https://github.com/project-chip/connectedhomeip/pull/27553/files#r1260287402

[bzbarsky-apple]

@lpbeliveau-silabs For future reference, links to comments in the "files" view go dead when the code changes... You probably want:

• [ReadHandler] Report Scheduler class #27553 (comment)
• [ReadHandler] Report Scheduler class #27553 (comment)

to link to the comments in the main discussion view... (note that the comment ids are the same).

[lpbeliveau-silabs]

Solved by #28657