remove / minimise synchronization locks between alignment workers and alignment data

In previous versions of Jalview, alignment calculations like consensus and conservation took place concurrently to any user operations, so someone can continue to explore/mark up an alignment whilst the conservation and consensus panels are being updated.

In 2.10, the basic alignment calculation workers (consensus, conservation, etc) seem to block further interactions, so whilst a calculation is in progress, the UI layer is still working but some events will block (e.g. column selection, edits).