Reduced slow tasks and memory consumption in East_Asia

[micheles]

Part of #11559. Here are the figures for Japan on engine192:

# before
| calc_12, maxmem=299.5 GB     | time_sec | memory_mb | counts    |
|------------------------------+----------+-----------+-----------|
| total ebrisk                 | 487_498  | 895.1     | 1_879     |
| computing risk               | 426_721  | 0.0       | 401_537   |
| aggregating losses           | 25_695   | 0.0       | 401_537   |
| EventBasedRiskCalculator.run | 12_305   | 1_721     | 1         |

# after
| calc_40, maxmem=307.4 GB     | time_sec | memory_mb | counts  |
|------------------------------+----------+-----------+---------|
| total ebrisk                 | 468_774  | 1_105     | 276     |
| computing risk               | 420_695  | 0.0       | 278_725 |
| aggregating losses           | 24_908   | 0.0       | 278_725 |
| EventBasedRiskCalculator.run | 12_412   | 1_827     | 1       |

The improvement is really minor for Japan since the slow tasks are the same:

| operation-duration | counts | mean  | stddev |   min | max    | slowfac |
|--------------------+--------+-------+--------+-------+--------+---------|
| ebrisk             |    159 | 3_058 |    87% | 323.5 | 12_059 |  3.9436 |
| ebrisk             |    157 | 2_976 |    89% | 288.6 | 12_182 |  4.0935 |

However, in small calculations, we can double the speed of the risk part, for instance for slovenia.zip:

| calc_7973, maxmem=9.8 GB     | time_sec | memory_mb | counts |
|------------------------------+----------+-----------+--------|
| computing risk               | 22.4886  | 0.0       | 4_407  |
| computing risk               | 12.8242  | 0.0       | 2_227  |

Taiwan is even better, 3x for the slowest task:

| operation-duration | counts | mean   | stddev | min     | max     | slowfac |
|--------------------+--------+--------+--------+---------+---------+---------|
| ebrisk             | 185    | 416.1  | 45%    | 43.1735 | 1_620   | 3.8938  |
| ebrisk             | 166    | 395.2  | 23%    | 40.1724 | 596.8   | 1.5099  |

Also, there is a substantial memory saving on small machines: on cassidy Japan runs using ~72 GB or RAM, while before it was going OOM.