Allow to disable gap creation during block import (#5343)

This feature is helpful for us with custom sync protocol that is similar
to Warp sync except we do not ever sync the gap and don't want it to
exist in the first place (see
https://github.com/paritytech/polkadot-sdk/issues/5333 and its
references for motivation).

Otherwise we had to resort to this:
https://github.com/autonomys/polkadot-sdk/commit/d5375125ca7c59fcc1ac72dc1b2ac251cbc80323

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Co-authored-by: Davide Galassi <[email protected]>