Faster SQL implementation in d6stack library
#### Code Sample

https://github.com/d6t/d6tstack/blob/master/examples-sql.ipynb

#### Problem description

In the example it currently takes 28s to upload data to an SQL database. With the d6stack implementation it only takes 4.7s with postgres and 7.1s with mysql which is 4 to 6 times faster than standard pandas.

#### Expected Output

Improved speed uploading data to an SQL database right out of the box.