[Rust][DataFusion]Use Timestamp(Nanosecond, None) for SQL TIMESTAMP Type

[asfimport]

1. Rationale
   Running the query CREATE EXTERNAL TABLE .. (c TIMESTAMP) today in DataFusion will result in a data type pf "Date64" which means that anything more specific than the date will be ignored.

   This leads to strange behavior such as

   echo "Jorge,2018-12-13T12:12:10.011" >> /tmp/foo.csv
   echo "Andrew,2018-11-13T17:11:10.011" > /tmp/foo.csv
   
   cargo run -p datafusion --bin datafusion-cli
       Finished dev [unoptimized + debuginfo] target(s) in 0.23s
        Running `target/debug/datafusion-cli`
   > CREATE EXTERNAL TABLE t(a varchar, b TIMESTAMP)
   STORED AS CSV
   LOCATION '/tmp/foo.csv';
   
   0 rows in set. Query took 0 seconds.
   > select * from t;
   +--------+------------+
   | a      | b          |
   +--------+------------+
   | Andrew | 2018-11-13 |
   | Jorge  | 2018-12-13 |
   +--------+------------+

   (note how it is only a date, not a timestamp)

Reporter: Andrew Lamb / @alamb
Assignee: Andrew Lamb / @alamb

PRs and other links:

• GitHub Pull Request #9936

_{Note: This issue was originally created as ARROW-12278. Please see the migration documentation for further details.}

[asfimport]

Andrew Lamb / @alamb:
Issue resolved by pull request 9936
#9936