Support import of category type in DB Engine

For following script

import pandas as pd
import pyarrow as pa
import sys
sys.setdlopenflags( 1|256 )    # RTLD_LAZY+RTLD_GLOBAL
from dbe import PyDbEngine as PDE

pdb = PDE("data", 8874)
data = {
        "id1": ["id1", "id2", "id3", "id1", "id2", "id3", "id1", "id2", "id3", "id1"],
    }
df = pd.DataFrame(data)
df["id1"] = df["id1"].astype("category")
a = pa.Table.from_pandas(df)
pdb.consumeArrowTable('testtable', a)

we currently have

terminate called after throwing an instance of 'std::runtime_error'
  what():  dictionary<values=string, indices=int8, ordered=0> is not yet supported.
Aborted (core dumped)

this is h2o case, reproducible on https://github.com/intel-go/omniscidb/commits/consuming_arrow_table

[Garra1980]

For now in case of import with arrow we can just omit conversion to category since FSI treats strings as encoded dictionary anyway

[fexolm]

@anmyachev we are waiting a small fix from omnisci team. See https://github.com/modin-project/modin/issues/2747.