added new column 'updated' for realestate
renamed column 'link' to 'hlid' in 'community'
added new column 'subdist' in table 'realestate'

it appears that estate and subdist is a 1-1 relation.
prepare to merge zoning to realestate.

will drop table zoning later.
added estate and cummunity detail crawler

added centralized info retrieval function
replaced try/except with get_or_create()
added multi-process support for estate crawler