Make sessions threadsafe

I ran into the thread safety issue described here:

#153

According to boto3's docs, each thread should have its own session instance:

http://boto3.readthedocs.io/en/latest/guide/resources.html#multithreading

I resolved this by modifying the session property on the connection
class. It makes self._session into a thread local variable, and
attaches a session to self._session.session. The session property still
has the same behavior, but now it's creating a session on a
per-connection-per-thread basis instead of just on a per-thread basis.

This issue was intermittent to begin with, and I hadn't figured out a
way to reliably reproduce it, or I would have written a test. It still
passes PynamoDB's unit tests, and I haven't run into the thread safety
issue described in #153 since I've been running the patched version.

pynamodb/connection/base.py

@@ -7,6 +7,7 @@
 import math
 import random
 import time
+from threading import local
 
 import six
 from six.moves import range
@@ -181,7 +182,7 @@ def __init__(self, region=None, host=None, session_cls=None,
                  request_timeout_seconds=None, max_retry_attempts=None, base_backoff_ms=None):
         self._tables = {}
         self.host = host
-        self._session = None
+        self._session = local()
         self._requests_session = None
         self._client = None
         if region:
@@ -392,9 +393,9 @@ def session(self):
         """
         Returns a valid botocore session
         """
-        if self._session is None:
-            self._session = get_session()
-        return self._session
+        if getattr(self._session, "session", None) is None:
+            self._session.session = get_session()
+        return self._session.session
 
     @property
     def requests_session(self):