Reduced max_assets_chunk to reduce the memory occupation

[micheles]

Plus other minor changes. The effect is minor oin Taiwan:

# master
$ OQ_SAMPLE_ASSETS=.01 oq engine --run job_Taiwan.ini --hc ../SES.hdf5 
| calc_13, maxmem=183.6 GB     | time_sec | memory_mb | counts  |
|------------------------------+----------+-----------+---------|
| total ebrisk                 | 9_554    | 554.8     | 2_001   |
| computing risk               | 4_716    | 0.0       | 118_293 |
| reading sites                | 1_208    | 184.3     | 188     |
| reading ruptures             | 826.4    | 0.0       | 2_047   |
| EventBasedRiskCalculator.run | 537.2    | 691.2     | 1       |
| reading crmodel              | 501.9    | 369.0     | 188     |

| operation-duration | counts | mean    | stddev | min    | max    | slowfac |
|--------------------+--------+---------+--------+--------+--------+---------|
| ebrisk             | 188    | 50.9786 | 94%    | 9.1446 | 448.2  | 8.7914  |

# more_mem
$ OQ_SAMPLE_ASSETS=.01 oq engine --run job_Taiwan.ini --hc ../SES.hdf5 
| calc_14, maxmem=183.6 GB     | time_sec | memory_mb | counts  |
|------------------------------+----------+-----------+---------|
| total ebrisk                 | 9_204    | 554.7     | 2_001   |
| computing risk               | 4_720    | 0.0       | 118_292 |
| reading sites                | 888.1    | 184.2     | 188     |
| reading ruptures             | 723.0    | 0.0       | 2_047   |
| EventBasedRiskCalculator.run | 534.6    | 711.1     | 1       |
| reading crmodel              | 502.6    | 369.0     | 188     |

| operation-duration | counts | mean    | stddev | min    | max    | slowfac |
|--------------------+--------+---------+--------+--------+--------+---------|
| ebrisk             | 188    | 49.1032 | 99%    | 9.2018 | 446.7  | 9.0977  |

The improvement is in reading from the datastore since we are reading a bit less thanks to the introduction of

pairs = [(idx0taxo, slice(s0, s1))
                 for idx0taxo, s0, s1 in monitor.read('start-stop')]

moved outside the loop.