Improved build_global_exposure/3

[micheles]

Part of #9227. Here are the figures now

[#56292 INFO] Received 23277 * 568.52 KB {'tot': '12.62 GB'} in 334 seconds
<Monitor [michele], duration=495.04037261009216s, memory=19.91 GB>
| calc_56292, maxmem=28.6 GB | time_sec | memory_mb | counts |
|----------------------------+----------+-----------+--------|
| total exposure_by_geohash  | 1_679    | 20.1      | 5_624  |
| total gen_tasks            | 1_496    | 6.81943   | 17_745 |

| operation-duration  | counts | mean | stddev | min     | max   | slowfac |
|---------------------+--------+------+--------+---------+-------+---------|
| exposure_by_geohash | 92     | 18.3 | 37%    | 0.23760 | 31.8  | 1.73985 |
| gen_tasks           | 19     | 78.7 | 40%    | 27.9    | 172.0 | 2.18515 |