Fix #97: Concurrency Issue

The issue was there all the time, but became critical in 9950dd0 with version
0.3.3.

`class _S3ConfigurationMap` is not thread safe because both
`set/get_configuration()` ask for `self.arguments` and `self.resources`
being `None`. However, since the configuration map is a singleton, when
using the library in a multithreaded environment, it can happen that
half the part of `_initial_setup` has run and `self.arguments` is indeed
not `None`, but `self.resources` is still `None` because in another
thread, creation of the boto3 resource is still ongoing. Then, our
thread would *not* call `_initial_setup()`, but still die on
`self.resources` being `None` when trying to access it, leading to the
error described in #97.

I could not make the bug appear without introducing the IO delay by
creating the resource in `_initial_setup()`. Maybe it is a CPython
implementation detail that makes `_initial_setup()` atomic or incredibly
rare to not be atomic without the creation of the boto resource.

Anyways, the setup is now synchronised with a lock, which is required,
again, given that `_S3ConfigurationMap` is a singleton.

Here is the code that reliably triggers the bug without the change:

```py
from multiprocessing.pool import ThreadPool

from s3path import S3Path

def do(i):
    dst = S3Path.from_uri('s3://something') / f'hello_{i}.txt'
    dst.write_bytes(b'hello')

ThreadPool(processes=20).map(do, range(100))
```

s3path.py

@@ -10,6 +10,7 @@
 from distutils.version import StrictVersion
 from io import DEFAULT_BUFFER_SIZE, UnsupportedOperation
 from pathlib import _PosixFlavour, _Accessor, PurePath, Path
+from threading import Lock
 
 try:
     import boto3
@@ -58,14 +59,20 @@ def __init__(self, default_resource_kwargs, **default_arguments):
         self.default_arguments = default_arguments
         self.arguments = None
         self.resources = None
+        self.setup_lock = Lock()
+        self.is_setup = False
 
     @property
     def default_resource(self):
         return boto3.resource('s3', **self.default_resource_kwargs)
 
-    def _initial_setup(self):
-        self.arguments = {PureS3Path('/'): self.default_arguments}
-        self.resources = {PureS3Path('/'): self.default_resource}
+    def _delayed_setup(self):
+        """ Resolves a circular dependency between us and PureS3Path """
+        with self.setup_lock:
+            if not self.is_setup:
+                self.arguments = {PureS3Path('/'): self.default_arguments}
+                self.resources = {PureS3Path('/'): self.default_resource}
+                self.is_setup = True
 
     def __repr__(self):
         return '{name}(arguments={arguments}, resources={resources})'.format(
@@ -74,17 +81,15 @@ def __repr__(self):
             resources=self.resources)
 
     def set_configuration(self, path, *, resource=None, arguments=None):
-        if self.arguments is None and self.resources is None:
-            self._initial_setup()
+        self._delayed_setup()
         if arguments is not None:
             self.arguments[path] = arguments
         if resource is not None:
             self.resources[path] = resource
 
     @lru_cache()
     def get_configuration(self, path):
-        if self.arguments is None and self.resources is None:
-            self._initial_setup()
+        self._delayed_setup()
         resources = arguments = None
         for path in chain([path], path.parents):
             if resources is None and path in self.resources: