Join-diff (in-db) + new query builder

[erezsh]

Replacing #236

Big PR containing -

• Join-diff (in-db) implementation of data-diff, with collection of extra statistics, working for all major databases
• Query builder package, for easy construction of SQL queries with Python. While not ready yet for all use-cases, it's complete enough for the data-diff use-case.
• Existing differ moved to "hashdiff_tables" module, and updated to use the new query builder
• Updated the CLI and Python API to allow using both algorithms.
• Schema diff when comparing in same db

New CLI interface works as follow:

data-diff db1 table1 db2 table2    # hashdiff (just as before)
data-diff db1 table1 table2        # joindiff
data-diff db1 table1 db2 table2  -a joindiff  # also joindiff, throws error if db1 != db2

TODO:

• More tests for the joindiff algorithm
• Update readme docs