Support joins from sqlite tables in tile-join

[e-n-f]

This PR adds options to tile-join to join attributes from a sqlite3 database onto existing tiled geometries. The new options are:

• --join-sqlite file.db
• --join-tile-attribute attribute: the name of the attribute in the tiled features that is used for the join key
• --join-table-expression expression: the sql expression that is matched in the database table: for example lower(country) to match lowercase country names in the tiles against mixed-case country names in the database by smashing their case
• --join-table table: the name of the table to be matched against in the database
• --join-count-limit limit: the maximum number of rows from the database (default 1) that will be joined to copies of the feature in the tiles
• --use-attribute-for-id attribute: use the named attribute from the sql row as the feature ID, instead of the feature ID from the tiled feature that it is joined to.

It also improves tile-joins bounding box output so that the bounding box reflects the bounds of all copied features at all zoom levels rather than only the bounding box of the tiles from which any features were copied at maxzoom. (The zoom-neutrality is necessary to play nicely with --generate-variable-depth-tile-pyramid.)