Improved build_global_exposure/5

[micheles]

Part of #9227. The previous PR was missing some assets. Here are the right figures:

Received 21919 * 533.02 KB {'tot': '11.14 GB'} in 275 seconds
<Monitor [michele], duration=432.1746280193329s, memory=15.46 GB>

| calc_56325, maxmem=27.4 GB | time_sec | memory_mb | counts |
|----------------------------+----------+-----------+--------|
| total gen_tasks            | 1_052    | 1.39588   | 18_375 |
| total exposure_by_geohash  | 639.2    | 0.42107   | 3_585  |

| operation-duration  | counts | mean | stddev | min     | max  | slowfac |
|---------------------+--------+------+--------+---------+------+---------|
| exposure_by_geohash | 41     | 15.6 | 37%    | 0.09763 | 23.3 | 1.49761 |
| gen_tasks           | 19     | 55.4 | 31%    | 17.9    | 85.9 | 1.55067 |