Blobfuse Flexvol high memory consumption

**What happened**:
I am using AKS to host Azure Machine Learning deployments. Azure Machine Learning installs Blobfuse Flexvol on all nodes and uses it in the pods it deploys.
After a while I noticed on all nodes where pods are using blobfuse, available memory was low. Looking at the processes I noticed the blobfuse process taking up a large amount of memory, increasing over time:
```

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
70814 root 20 0 1584.4m 1.020g 3.9m S 1.0 15.0 111:44.20 blobfuse /var/lib/kubelet/pods/7eb4e9f6-b24c-409d-acdc-d6c7c3245db1/volumes/azure~blobfuse/coguk1-0-acc-acc72c4e19281934d378bc07280e08bb043-pvc --container-name=azureml --tmp-path=/tmp/blobfuse/ -o allow_other -o ro --file-cache-timeout-in-seconds=1200
70820 root 20 0 1392.4m 908.8m 1.4m S 1.0 13.1 111:21.89 blobfuse /var/lib/kubelet/pods/1b1f33d2-da0b-4c64-bc38-9a766689b36a/volumes/azure~blobfuse/cogukag-acc-accd16a35e863fc41c79976f144ce3100e7-pvc --container-name=azureml --tmp-path=/tmp/blobfuse/ -o allow_other -o ro --file-cache-timeout-in-seconds=1200
70893 root 20 0 1264.4m 859.4m 2.0m S 0.7 12.4 111:34.31 blobfuse /var/lib/kubelet/pods/d6fac6a9-9718-43d9-b088-e2162ba7a2ef/volumes/azure~blobfuse/cogusdbm-acc-accade73fcaa67b487fa0ed702af4305f82-pvc --container-name=azureml --tmp-path=/tmp/blobfuse/ -o allow_other -o ro --file-cache-timeout-in-seconds=1200
70991 root 20 0 1264.4m 740.5m 3.8m S 1.0 10.7 111:33.34 blobfuse /var/lib/kubelet/pods/b713a5bf-f9cb-46c0-8f20-1630c50ff7eb/volumes/azure~blobfuse/cogex-acc-acc24e1419c60844136b765328478223650-pvc --container-name=azureml --tmp-path=/tmp/blobfuse/ -o allow_other -o ro --file-cache-timeout-in-seconds=1200
```
It doesn't seem the memory usage ever drops.

**What you expected to happen**:
memory usage going up and down based on usage.

**Anything else we need to know?**:
not sure

**Environment**:
- Kubernetes version (use `kubectl version`):
Client Version: version.Info{Major:"1", Minor:"16", GitVersion:"v1.16.8", GitCommit:"ec6eb119b81be488b030e849b9e64fda4caaf33c", GitTreeState:"clean", BuildDate:"2020-03-12T21:00:06Z", GoVersion:"go1.13.8", Compiler:"gc", Platform:"windows/amd64"}
Server Version: version.Info{Major:"1", Minor:"16", GitVersion:"v1.16.10", GitCommit:"89d8075525967c7a619641fabcb267358d28bf08", GitTreeState:"clean", BuildDate:"2020-06-23T02:52:37Z", GoVersion:"go1.13.9", Compiler:"gc", Platform:"linux/amd64"}

- OS (e.g. from /etc/os-release):
NAME="Ubuntu"
VERSION="16.04.6 LTS (Xenial Xerus)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 16.04.6 LTS"
VERSION_ID="16.04"

- Kernel (e.g. `uname -a`):
Linux aks-agentpool-38312370-vmss000002 4.15.0-1091-azure #101~16.04.1-Ubuntu SMP Fri Jun 26 01:14:12 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

- Install tools:

- Others:
blobfuse FlexVolume driver 1.0.13

Start by reproducing the memory growth with blobfuse FlexVolume driver 1.0.13 in the reported AKS, Kubernetes 1.16, and Ubuntu 16.04 environment. Monitor the blobfuse processes and compare memory against filesystem usage; done means identifying the cause of the persistent growth and documenting or fixing the behavior.

Written by the indexing model from the issue text.