Optimize BQ uploads by reusing tables (cache)

[ljvmiranda921]

We upload a dataframe into BQ (as a table) for every call. This is inefficient given larger datasets. There should be a better way to:

• Check if the dataframe in question already exists in the BigQuery dataset
• If yes, then just get that table, else, do the upload.

[marksteve]

@ljvmiranda921 https://pandas-docs.github.io/pandas-docs-travis/reference/api/pandas.util.hash_pandas_object.html