read_csv s3 file parameter memory_map default is True?
#### Code Sample

```python
# code 1
def get_pid():
    info = pd.read_csv('s3://vision.algo.data/cyz/tmp/test/11.csv', header=None, names=['pid'])
    print(f'pid num : {len(info)}')

# code 2
def get_pid():
    info = pd.read_csv('s3://vision.algo.data/cyz/tmp/test/11.csv', header=None, names=['pid'], memory_map=False)
    print(f'pid num : {len(info)}')

while True:
    get_pid()
    time.sleep(4)
```
#### Problem description
when I use `code1`, if s3 file changed, `info` not change.
`code2` can find the change.
The difference between these two code is parameter `memory_map`, actually, default `memory_map`  is False,  so I was confused. 


#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.4.0-1098-aws
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8
pandas           : 0.25.3
numpy            : 1.17.4
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 19.3.1
setuptools       : 42.0.2
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.4 (dt dec pq3 ext lo64)
jinja2           : 2.10.3
IPython          : 7.10.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : 0.4.0
scipy            : 1.3.3
sqlalchemy       : 1.3.11
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
</details>
