OBS_GetMeasure performance improvements

[rafatower]

Just to follow-up on the performance metrics and findings here: https://github.com/CartoDB/dataservices-api/issues/306#issuecomment-264183386

Proposed changes:

• Avoid passing literal geometries in dynamic calls (sql = format('%L...', geom) and then EXECUTE sql)
• Avoid too complex features in ST_Intersection by using ST_Subdivide and/or ST_Simplify.

Work being done here: https://github.com/CartoDB/observatory-extension/compare/remove-format-literals

Side note: watch out for validity of geometries, especially taking into account these lines (snap to grid and buffers): https://github.com/CartoDB/observatory-extension/blob/cbe7b6dd156554abebce59282e77a62037de0d29/src/pg/sql/41_observatory_augmentation.sql#L372-L382 cc/ @rochoa

[talos]

Additional changes:

• Optimize _OBS_GetGeometryScores, as the commit which added it (17d343a756c7356ce085060b80040d21398d8544) caused a major regression in performance (see below)
• Add functions that Camshaft can take advantage of to call the metadata function (ergo the rasters) once for a set of geometries, as opposed to once for each geometry

[talos]

We're now using OBS_GetData in the UI, and OBS_GetMeasure is doing about as well as it can within the limitations of dynamically choosing geometries each time (if user doesn't choose a geom.)