Fix a few memory issues found by ASAN

[sfc-gh-anoyes]

When using an external client, it's actually the ThreadFuture object (and not
the contained Standalone) that owns the underlying memory. Extend the lifetime
of some ThreadFuture's in TenantCommands.actor.cpp to accommodate this confusing
situation. Ideally we find a better way to deal with this, but this is what we
have right now.

Also call fdb_tenant_destroy in the unit tests.

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[fdb-windows-ci]

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• Commit ID: e0618cb
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[foundationdb-ci]

AWS CodeBuild CI Report for Linux CentOS 7

• CodeBuild project: foundationdb-pr
• Commit ID: e0618cb
• Result: FAILED
• Error: Error while executing command: if python3 -m joshua.joshua list --stopped | grep ${ENSEMBLE_ID} | grep -q 'pass=10[0-9][0-9][0-9]'; then echo PASS; else echo FAIL && exit 1; fi. Reason: exit status 1
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[fdb-windows-ci]

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[foundationdb-ci]

AWS CodeBuild CI Report for macOS BigSur 11.5.2

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[foundationdb-ci]

AWS CodeBuild CI Report for Linux CentOS 7

• CodeBuild project: foundationdb-pr
• Commit ID: d727e76
• Result: SUCCEEDED
• Error: N/A
• Build Logs (available for 30 days)