Ability to configure SQLite cache_size

[simonw]

See https://www.sqlite.org/pragma.html#pragma_cache_size

Let's call the config setting cache_size_kb to emphasize that we're using the negative option.

Note this warning: perhaps we should raise an error if you try to use this setting against a SQLite version prior to 3.7.10

If the argument N is positive then the suggested cache size is set to N. If the argument N is negative, then the number of cache pages is adjusted to use approximately abs(N*1024) bytes of memory. Backwards compatibility note: The behavior of cache_size with a negative N was different in prior to version 3.7.10 (2012-01-16). In version 3.7.9 and earlier, the number of pages in the cache was set to the absolute value of N.

[simonw]

I prototyped this a while ago here 04476ea before we had the --config mechanism.

[simonw]

In the interest of getting this shipped, I'm going to ignore the 3.7.10 issue.

[simonw]

Docs: http://datasette.readthedocs.io/en/latest/config.html#cache-size-kb