Use read lock for QuotaMonitor#IsSynced

[tedyu]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
QuotaMonitor#IsSynced traverses qm.monitors where holding read lock is enough.

NONE

[k8s-ci-robot]

Hi @tedyu. Thanks for your PR.

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[neolit123]

/ok-to-test
/priority backlog

[tedyu]

ping @derekwaynecarr for review

thanks

[tedyu]

/assign @deads2k

[tedyu]

ping @mikedanese @janetkuo for review

[smarterclayton]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: smarterclayton, tedyu

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