Can now handle several additional Postgres functions to get a tile, plus
tons of small fixes
### Multiple result variants
* `getmvt(z,x,y) -> [bytea,md5]` (single row with two columns)
* `getmvt(z,x,y) -> [bytea]` (single row with a single column)
* `getmvt(z,x,y) -> bytea` (value)
### Multiple input parameter variants
* `getmvt(z, x, y)` or `getmvt(zoom, x, y)` (all 3 vars must be
integers)
* `getmvt(z, x, y, url_query)`, where instead of `url_query` it could be
any other name, but must be of type JSON
### Breaking
* srid is now the same type as PG -- `i32`
* renamed config vals `table_sources` and `function_sources` into
`tables` and `functions`
### Features and fixes
* if postgis is v3.1+, uses margin parameter to extend the search box by
the size of the buffer. I think we should make 3.1 minimal required.
* fixes feature ID issue from #466
* fixes mixed case names for schemas, tables and columns, functions and
parameter names per #389
### Notes
* More dynamic SQL generation in code instead of using external SQL
files. Those should only be used when they are not parametrized.
* The new function/table discovery mechanism: query for all functions in
the database, and match up those functions with the ones configured (if
any), plus adds all the rest of the un-declared ones if discovery mode
is on.
* During table and function discovery, the code generates a map of
`(PgSqlInfo, FunctionInfo)` (or table) tupples containing SQL needed to
get the tile.
* Auto-discovery mode is currently hidden - the discovery is on only
when no tables or functions are configured. TBD - how to configure it in
the future
* The new system allows for an easy way to auto-discover for the
specific schemas only, solving #47
* predictable order of table/function instantiation
* bounding boxes computed in parallel for all tables (when not
configured)
* proper identifier escaping
* test cleanup
fixes#378fixes#466fixes#65fixes#389
* All tests and internal code now uses ST_TileEnvelope function
* Remove `tile_bbox`
* Rename test function sources for clarity - this will be needed in a
subsequent PR to add other function tests
