Remove calls to grpc.channel.close(), which could cause segfaults (#266)

## What is the goal of this PR?

We no longer call `close` on any of our gRPC Channels. This fixes
possible segfaults caused by resources being deallocated while they are
still in use.

## What are the changes implemented in this PR?

Users in a wide variety of scenarios had reported intermittent crashes,
often accompanied by warnings saying "1 metadata element(s) leaked". The
logs would look similar to the following:
```
7786| E1031 17:59:35.477338904 207 metadata.cc:253] WARNING: 1 metadata elements were leaked
7787| E1031 17:59:35.477409424 207 metadata.cc:260] mdelem ':authority' = 'typedb-cluster-1.typedb-cluster:1729'
7788| [mutex.cc : 435] RAW: Lock blocking 0x563cbf829fc0 @
7789| [mutex.cc : 1908] RAW: Check (v & (kMuWait | kMuWrWait)) != kMuWrWait failed: Lock: Mutex corrupt: waiting writer with no waiters: 0x563cbf815670
```
The same issue has also been reported in
googleads/google-ads-python#384, and a fix was
suggested in:
- grpc/grpc#19235

From this issue we infer that `Channel.close` is not behaving nicely in
gRPC Python, and can cause resources to be deallocated while they are
still in use. As gRPC itself uses native C libraries, this results in
segfaults and crashes. We determine that the best course of action is to
not close the Channel ourselves.

We've simply deleted the 3 places in our code that called `close` on a
gRPC `Channel`. It has passed our CI tests and fixed user-reported
issues, and it is what the gRPC maintainers themselves appear to
recommend in the linked `grpc` issue.

- closes #239 
- see #265

Author: alexjpwalker

.grabl/automation.yml renamed to .factory/automation.yml

@@ -50,7 +50,7 @@ checkstyle_test(
     include = glob([
         ".bazelrc",
         ".gitignore",
-        ".grabl/*",
+        ".factory/*",
         "BUILD",
         "WORKSPACE",
         "deployment.bzl",

.grabl/test-cluster.sh renamed to .factory/test-cluster.sh

@@ -63,7 +63,6 @@ def _new_channel(self) -> grpc.Channel:
 
     def close(self) -> None:
         super().close()
-        self._channel.close()
 
 
 class _CredentialAuth(grpc.AuthMetadataPlugin):

.grabl/test-core.sh renamed to .factory/test-core.sh

@@ -49,4 +49,3 @@ def new_channel_and_stub(self) -> (Channel, _CoreStub):
 
     def close(self) -> None:
         super().close()
-        self._channel.close()

BUILD

@@ -101,7 +101,6 @@ def rollback(self):
 
     def close(self):
         self._bidirectional_stream.close()
-        self._channel.close()
 
     def __enter__(self):
         return self