store: Give context deadline for flush operation in `flushLoop`

Nice, we should actually do the same in the flushLoop in the store.

_Originally posted by @Wondertan in https://github.com/celestiaorg/go-header/pull/52#discussion_r1240223951_

Locate the store's flushLoop and inspect the existing flush operation that this issue says should be handled the same way. Determine where the context deadline belongs, then verify that the flush operation observes it and add or update coverage if the surrounding code provides a relevant test entry point.

Written by the indexing model from the issue text.