Modified script to commit smaller batches of symbols to the database.

Summary:
Modified the import script so it will only try to load a configurable
number of symbols at a time to avoid exhausting memory for large project
imports.

I haven't written a line of PHP in more than a decade, so please forgive
any stylistic or technical errors.

Test Plan: Ran the script on symbol table generated from linux kernel.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4117

Differential Revision: https://secure.phabricator.com/D7596