Support multiple adaptors in CLI and worker

[josephjclark]

The Collections API requires us to support multiple adaptors in the Worker (because we use an adaptor to provide the collections API).

The runtime and compiler should support multiple adaptors, they don't much care. And I think the CLI right now is cool with it.

Worker and Engine probably just assume a single adaptor.

So what I think we need to do is:

• Lightning Runs can include an array of adaptors (although they won't, apart from unit tests)
• ExecutionPlans should use an array of adaptors for each step (even if it's an array of 1)
• Ensure that this all works OK
• Ensure that the compiler can correctly generate imports with multiple adaptors (this could be a gotcha - didn't we switch off "smart imports"?)
• Create multi-adaptor integration tests in CLI and Worker

[josephjclark]

Done in #801