Fix a hang upon pause/resume.

[mfelsche]

The cause was that upon resume we only woke 2 of the maximum 3 quiescence beacons (1 for ConnectorContext, 1 for SourceContext, 1 for SinkContext).
I did miss that we actually clone it three times instead of 2.

Pull request

Checklist

• The RFC, if required, has been submitted and approved
• Any user-facing impact of the changes is reflected in docs.tremor.rs
• The code is tested
• Use of unsafe code is reasoned about in a comment
• Update CHANGELOG.md appropriately, recording any changes, bug fixes, or other observable changes in behavior
• The performance impact of the change is measured (see below)

Performance

Shouldn't have any impact

[darach]

LGTM 👍

[coveralls]

Pull Request Test Coverage Report for Build 2159915354

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 6 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.002%) to 85.394%

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Files with Coverage Reduction	New Missed Lines	%
src/connectors.rs	1	85.89%
src/system/flow.rs	1	89.9%
src/connectors/source.rs	4	81.24%

Totals
Change from base Build 2156195923:	-0.002%
Covered Lines:	39534
Relevant Lines:	46296

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💛 - Coveralls