Allow skipping utf8 converison in Python3

[csringhofer]

Ran some benchmarks on local Impala dev environment:
Query: select * from tpch_parquet.lineitem"
returns 6 million rows (numbers, strings)

ClientFetchWaitTimer looks like this:
Impyla (Python2, fetchcolumnar()) 7s967ms
Impyla (Python3, fetchcolumnar()): 14s163ms
Impyla (Python2, fetchmany(1024)): 17s791ms
Impyla (Python3, fetchmany(1024)): 23s816ms

Based on cProfile the Python2 vs 3 differences come from _convert_strings_to_unicode().

Adding a cursor option like "convert_strings_to_unicode" similarly to "convert_types" would be useful.