Set default parallelism of CoreClient to 2 (#211)

## What is the goal of this PR?

Given that Python doesn't provide an easy way of getting the number of CPUs available to the process, we decided to reduce the default parallelism of CoreClient from 3 to 2. This should ensure it continues to run smoothly on systems with fewer cores.

## What are the changes implemented in this PR?

Set default parallelism of CoreClient to 2

Author: Alex Walker

grakn/client.py

@@ -32,11 +32,11 @@ class Grakn:
     DEFAULT_ADDRESS = "localhost:1729"
 
     @staticmethod
-    def core_client(address: str, parallelisation: int = None) -> GraknClient:
+    def core_client(address: str, parallelisation: int = 2) -> GraknClient:
         return _CoreClient(address, parallelisation)
 
     @staticmethod
-    def cluster_client(addresses: Union[Iterable[str], str], parallelisation: int = None) -> GraknClusterClient:
+    def cluster_client(addresses: Union[Iterable[str], str], parallelisation: int = 2) -> GraknClusterClient:
         if isinstance(addresses, str):
             return _ClusterClient([addresses], parallelisation)
         else:

grakn/core/client.py

@@ -29,16 +29,10 @@
 from grakn.stream.request_transmitter import RequestTransmitter
 
 
-def calculate_parallelisation():
-    # TODO: cores = Runtime.getRuntime().availableProcessors()
-    return 3
-
-
 class _CoreClient(GraknClient):
 
-    def __init__(self, address: str, parallelisation: int = None):
-        if parallelisation is None:
-            parallelisation = calculate_parallelisation()
+    # TODO: Detect number of available CPUs
+    def __init__(self, address: str, parallelisation: int = 2):
         self._address = address
         self._channel = insecure_channel(self._address)
         self._stub = GraknCoreStub(self._channel)