Add back the ability to blocking flush after a bootstrap

[robskillington]

The following PR works around an issue we encountered when naively trying to flush after a bootstrap:
#268

It does this by simply waiting for the next tick and a flush to occur to actually finish the bootstrap process.

Ideally we do actually flush directly after a bootstrap, however we need to coordinate with the ticking procedure to ensure that all buffers have been rotated into each series by waiting for a tick to rotate all the buffers before we can start a flush cycle (otherwise me miss blocks that are just about to rotate in for that time window when we flush and we don't flush them again because the time window is marked flushed).