SQLiteDatabaseClient schema methods

[mbostock]

This implements the describeTables and describeColumns methods for SQLite.

Fixes #288.

[mbostock]

Asking @visnup for another opinion here on how to pass through the raw database type.

[visnup]

I like what's here plus a single key called maybe databaseType or vendorType that includes the vendor-specific name for the type. And I'd want to re-work the schema object coming from data-connector to match this too, replacing the int32: true, float32: true entries with something like databaseType: "int32"

The only other thing worth considering is calling out bigint and date since the original JSON schema specification on ajv didn't provide for either. These were {type: "string", [bigint|date]: true} since over the wire these values were serialized as strings.

It seems since then there's JSON Type Definition which seems more promising?? And this more recent JSON Schema reference seems to document types for datetime.

[mbostock]

I’ve populated the databaseType field. Here’s what else I would like to do, but does not appear possible with our SQLite implementation, sql.js:

• Return a query result schema on db.query. The exec function doesn’t return any schema information other than the column names, so we don’t know what the types of the returned columns are. We’re populating the columns field but that’s it.
• Coerce date strings to Date instances. SQLite has DATE and DATETIME types, but the returned query results for these types are strings. Since we lack type information on the query results (previous point), we can’t know which columns are dates and then coerce them. We could map the DATE and DATETIME types to strings on describe, but… I dunno if that’s better.

We could probably fork sql.js to fix these issues.

[annie]

since this is a Table cell blocker, i think we should merge this once we've finalized the DatabaseClient schema API (based on Wiltse's PR) and updated this PR to match that specification (i've got a draft PR in the wings to make those updates).

it seems reasonable to me to address the query results schema and date string issues as fast follows, if that sounds good? i can create separate issues for those!

[mbostock]

FYI, the describeColumns query had a syntax error which I fixed in d381b56a159498f84e1e4fe61c397eb7d2732878.