Ensure Mnesia is running when Khepri is used as main metadata store

[gomoripeti]

Proposed Changes

This plugin still uses a pair of Mnesia tables. If not running, Mnesia is started in a single-node cluster mode. This is enough, as the used tables are not replicated.

(As discussed in #291)

CT tests to be added.

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[michaelklishin]

This version does work with Khepri enabled. I am testing an "upgrade" to Khepri next.

[michaelklishin]

A switch to Khepri with a node restart works as expected. Thank you, @gomoripeti.