[get_df] Adding support for multi-statement SQL

[john-bodley]

I'm working part of an Airbnb project to which leverages a custom SQLAlchemy dialect executes multiple-statements as it needs to create a series of temporary views before issuing the final select.

Many engines don't support multiple statements, i.e., one cannot provide a single string containing multiple-statements separated by the ; character and thus the only viable option seems to be to to mutate get_df to execute each statement separately using the same connection. Only the results from the last statement are fetched.

@mistercrunch I'm not sure how you feel about this change as it does not directly benefit any existing Superset use cases. I'm definitely open to other suggestion, though I sense this is fairly benign.

to: @GabeLoins @michellethomas @mistercrunch

[codecov-io]

Codecov Report

Merging #5060 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5060      +/-   ##
==========================================
+ Coverage   77.52%   77.52%   +<.01%     
==========================================
  Files          44       44              
  Lines        8707     8709       +2     
==========================================
+ Hits         6750     6752       +2     
  Misses       1957     1957

Impacted Files	Coverage Δ
superset/models/core.py	86.52% <100%> (+0.04%)	⬆️

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[mistercrunch]

We're looking at allowing multi-statements in SQL Lab as well soon and will probably do this using sqlparse as any SQL parsing is somewhat brittle, especially when allowing free-form SQL.

We'll also probably at that time remove the reliance on pd.read_sql_query as it doesn't allow us the level of control we need for SQL Lab, and we'd like to have a common SQL execution path for both SQL Lab and Explore.

@betodealmeida FYI