Use native unnest() db function instead of explode_array()

Use native unnest() db function instead of explode_array()

Per LP 789747, the custom explode_array() database function
was found to generate significantly slower results for searches than
using the native (and equivalent) unnest() array function. Given that
unnest() has been part of PostgreSQL since 8.4 and Evergreen 2.0
requires a minimum of PostgreSQL 8.4, this patch removes the definition
of the explode_array() functions and replaces its usage with unnest().

Signed-off-by: Dan Scott <dscott@laurentian.ca>