grpc throws exceptions after sending message, even if message succeeds

**Detailed Description**
A clear and concise description of what the bug is.
grpc throws exceptions after sending message,

```
SEVERE: *~*~*~ Channel ManagedChannelImpl{logId=1, target=hcs.testnet.mirrornode.hedera.com:5600} was not shutdown properly!!! ~*~*~*
Make sure to call shutdown()/shutdownNow() and wait until awaitTermination() returns true.
java.lang.RuntimeException: ManagedChannel allocation site
at io.grpc.internal.ManagedChannelOrphanWrapper$ManagedChannelReference.(ManagedChannelOrphanWrapper.java:94)
at io.grpc.internal.ManagedChannelOrphanWrapper.(ManagedChannelOrphanWrapper.java:52)
at io.grpc.internal.ManagedChannelOrphanWrapper.(ManagedChannelOrphanWrapper.java:43)
at io.grpc.internal.AbstractManagedChannelImplBuilder.build(AbstractManagedChannelImplBuilder.java:512)
at com.hedera.hashgraph.sdk.mirror.MirrorClient.(MirrorClient.java:17)
at com.hedera.hcs.sxc.plugin.mirror.subscribe.MirrorTopicSubscriber.subscribe(MirrorTopicSubscriber.java:82)
at com.hedera.hcs.sxc.plugin.mirror.subscribe.MirrorTopicSubscriber.run(MirrorTopicSubscriber.java:74)
at java.base/java.lang.Thread.run(Thread.java:844)
```
**Actual Behavior**
To reproduce create an OutboundHCSMessage and send any message.
**Expected Behavior**
No grpc stack traces on stdout on successful messages.

**Environment:**
- Java: [e.g. OpenJDK 11.0.4]
- Node: [e.g. v12.9.1-x86]
- OS: [e.g. Ubuntu 18.04]
- Version: [e.g. 1.0.1]

**Additional Context**
Add any other context about the problem here. Attach any logs here, if applicable.