Multi ledger context - draft

[ghost]

This is a draft PR of multi-ledger support which contains some classes that may be useful. The issue is with the current test infrastructure. It does not subscribe and react to completion events, it was built with the sandbox in mind and needs a redesign in order to test an asynchronous distributed system. Currently there are race conditions that have only been mitigated by adding in sleep pauses.

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[digitalasset-cla]

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