[NPoS] Check if staker is exposed in paged exposure storage entries (#2369)

Addresses a bug caused by
https://github.com/paritytech/polkadot-sdk/pull/1189. The changes are
still not released yet, so would like to push the fix soon so it can go
together with the release of the above PR.

`fast_unstake` checks if a staker is exposed in an era. However, this fn
is still returning whether the staker is exposed based on the old
storage item. This PR fixes that by looking in both old and new exposure
storages.

Also adds some integrity tests for paged exposures.