bluez5: backend-native: make SCO acquire asynchronous + minor fixes

Don't block main loop in connect() for SCO sockets, as we can now do it asynchronously. Can be useful if connecting runs into problems.

See eg #3019 (comment 1758977)

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bluez5: don't try to acquire if we are getting errors

If transport goes into error state too often, fail instead of trying to acquire it again.

This avoids getting into a tight acquire->fail->reacquire loop.

See #3023 (closed)

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Plus minor fixes, see commits.