(PDB-810) WIP  Load test structured facts indices.

This patch addresses some performance issues for structured facts

- deletes the index of fact_values value_integer and value_float.  These
  indices were dead weight since a coalesce causes them to be disregarded.
  This is a stopgap solution to be augmented by PDB-811

- creates a trgm index on fact_values column value_string, so that our trgm
  indexes are now on fact_paths and value_string. This greatly speeds
  up queries on value_string and only moderately affects the table size. See
  https://gist.githubusercontent.com/wkalt/42792c53a5dc9f0840c4/raw/29f261316bc7e39d888250401f0b1f1c8c28a87b/gistfile1.txt
  for some comparisons on a 2000 node dataset with 100 facts per node.

- put a DISTINCT clause in our garbage collection on fact_paths.  This incurs a
  minor performance cost in small datasets, but yields a tremendous benefit in
  large and highly duplicated sets by reducing the load on the accompanying IN.
  (see https://gist.github.com/wkalt/09225b71588c35674827)

* still need to confirm that the depth index is used.