hygiene, introduce formal type mut_trans_t
hygiene, rename foff -> ftmap, also make mut_trans_t objects come back from getter
minor, slightly optimise bitte_hist() by using ...

the first ever recorded transaction to skip scanning
the initial portion of the timeline superfluously.
fix, actually allocate some memory for the 1st transaction array

... in the ftmap SoA.
Merge branch 'code/hygiene'

* code/hygiene:
  fix, actually allocate some memory for the 1st transaction array
  minor, slightly optimise bitte_hist() by using ...
  hygiene, rename foff -> ftmap, also make mut_trans_t objects come back from getter
  hygiene, introduce formal type mut_trans_t
minor, provide mut_stor_open() and mut_stor_close(), also ...

extend bitte API routines by mut_stor_t param.
test, adapt regression tests to additional stor argument
test, provide additional regressions to test store functionality
Merge branch 'core/backending'

* core/backending:
  test, provide additional regressions to test store functionality
  test, adapt regression tests to additional stor argument
  minor, provide mut_stor_open() and mut_stor_close(), also ...