*: compute rev before firing go routines

[xhebox]

Signed-off-by: xhe xw897002528@gmail.com

What problem does this PR solve?

Issue Number: close None

Problem Summary: Watch rev is computed after firing the go routine. That said another go routine may race and change e.kv.Rev() such that e.watch() is not watching the correct revision.

What is changed and how it works: Compute it outside the go routine.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change (Don't forget to add the declarative for API)
• Has weirctl change
• Other user behavior changes

Release note

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None