num_threads and cache_max_age should be --config options

[simonw]

datasette/datasette/app.py

Line 106 in 58b5a37

num_threads=3,

And

datasette/datasette/views/base.py

Line 325 in 58b5a37

r.headers["Cache-Control"] = "max-age={}".format(365 * 24 * 60 * 60)

Refs #275

[simonw]

I ran a very rough micro-benchmark on the new num_sql_threads config option.

datasette --config num_sql_threads:1 fivethirtyeight.db

Then

ab -n 100 -c 10 'http://127.0.0.1:8011/fivethirtyeight-2628db9/twitter-ratio%2Fsenators'

Number of threads	Requests/second
1	4.57
3	9.77
10	13.53
20	15.24
50	8.21

[simonw]

Documentation: http://datasette.readthedocs.io/en/latest/config.html#num-sql-threads