Correctly identify and clean taken-over sessions

[mochi-co]

An issue was highlighted in which the subscriptions for a client which has undertaken a session takeover could be cleared erroneously in #173.

This behaviour was caused by the orphaned client unsubscribing from the client's topics after the new client had already inherited the subscriptions and begun reading.

Additionally, I identified a secondary, more significant issue in which an expiring orphaned client would delete the existing client from the server client map.

This PR attempts to resolve this issue through three mechanisms:

1. By adding a new isTakenOver atomic bit switch to the client which is set to 1 when the client session is inherited by a new client and the client is orphaned (the pointer to the client has been, or shortly will be overwritten in the server client map).
2. The s.unsubscribeClient logic has been adjusted to operate in two steps: first, the client subscriptions are moved from the client itself. Then, unsubscribe from the server topics index is only performed if the client has not been taken over.
3. When an actively connected client connection expires, it may only be deleted from the server clients map if the client has not been taken over (i.e. it is the active, operating client).

Finally, the state of existing sessions is cleaned up on all inheritance to prevent sequential take-overs from increasing memory usage by inflights + subs * client-id.

This PR adds a new test case specifically to ensure client inheritance and state cleaning is performed as expected.

[coveralls]

Pull Request Test Coverage Report for Build 4267624673

• 31 of 31 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 98.594%

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Totals
Change from base Build 4247937237:	0.005%
Covered Lines:	5120
Relevant Lines:	5193

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💛 - Coveralls