fix: define and document `stop_gap`

fix: define and document `stop_gap`

- changes the code implementation to "the maximum number of consecutive unused addresses"
  in esplora async and blocking extjensions.
- treat `stop_gap = 0` as `stop_gap = 1` for all purposes.
- renames `past_gap_limit` to `gap_limit_reached` to indicate we want to break once the gap
  limit is reached and not go further in `full_scan`, as suggested in
  https://github.com/bitcoindevkit/bdk/issues/1227#issuecomment-1859040463
- change the tests according to the new implementation.
- add notes on what `stop_gap` means and links to convergent definition in other
  Bitcoin-related software.

Closes #1227