Reduce metadataLock contention in LedgerHandle

[lifepuzzlefun]

Descriptions of the changes in this PR:

Fix #4517

Motivation

we can move changeEnsembleLoop out side the metadataLock because the changing ensemble field can be a guard for do the action and the callback will trigger next.

Changes

(Describe: what changes you have made)

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[liangyepianzhou]

LGTM

[StevenLuMT]

Good jobs

[lhotari]

LGTM