Improved build_global_exposure/2

[micheles]

Part of #9227. Now the figures are

[#56285 INFO] Received 26508 * 497.53 KB {'tot': '12.58 GB'} in 330 seconds
<Monitor [michele], duration=493.16890954971313s, memory=21.13 GB>
| calc_56285, maxmem=29.4 GB | time_sec | memory_mb | counts |
|----------------------------+----------+-----------+--------|
| total exposure_by_geohash  | 1_911    | 35.7      | 10_113 |
| total gen_tasks            | 1_017    | 34.5      | 16_658 |

faster than before (493s < 574s).