"az mysql server-logs download" fails with "[Errno 104] Connection reset by peer" on large (>32MB) current-day logs

**Describe the bug**

**Command Name**

`az mysql server-logs download --name mysql-slow-$PAAS_HOST-2021061605.log --resource-group $PAAS_HOST_RG --server $PAAS_HOST`

**Errors:**
```
The command failed with an unexpected error. Here is the traceback:
[Errno 104] Connection reset by peer
Traceback (most recent call last):
File "/usr/lib64/az/lib/python3.6/site-packages/knack/cli.py", line 231, in invoke
cmd_result = self.invocation.execute(args)
File "/usr/lib64/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 657, in execute
raise ex
File "/usr/lib64/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 720, in _run_jobs_serially
results.append(self._run_job(expanded_arg, cmd_copy))
File "/usr/lib64/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 691, in _run_job
result = cmd_copy(params)
File "/usr/lib64/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 328, in __call__
return self.handler(*args, **kwargs)
File "/usr/lib64/az/lib/python3.6/site-packages/azure/cli/core/commands/command_operation.py", line 121, in handler
return op(**command_args)
File "/usr/lib64/az/lib/python3.6/site-packages/azure/cli/command_modules/rdbms/custom.py", line 574, in _download_log_files
urlretrieve(f.url, f.name)
File "/usr/lib64/python3.6/urllib/request.py", line 277, in urlretrieve
block = fp.read(bs)
File "/usr/lib64/python3.6/http/client.py", line 459, in read
n = self.readinto(b)
File "/usr/lib64/python3.6/http/client.py", line 503, in readinto
n = self.fp.readinto(b)
File "/usr/lib64/python3.6/socket.py", line 586, in readinto
return self._sock.recv_into(b)
File "/usr/lib64/python3.6/ssl.py", line 968, in recv_into
return self.read(nbytes, buffer)
File "/usr/lib64/python3.6/ssl.py", line 830, in read
return self._sslobj.read(len, buffer)
File "/usr/lib64/python3.6/ssl.py", line 587, in read
v = self._sslobj.read(len, buffer)
ConnectionResetError: [Errno 104] Connection reset by peer
```

**To Reproduce**

Find a MySQL PaaS host which generates a large "slow queries" log.
Download the currently updating log with the command above using the $PAAS_HOST variable for the PaaS host name in Azure.

**Expected behavior**

The requested log file will (eventually) be downloaded.

**Environment summary**

```
Linux-3.10.0-957.21.3.el7.x86_64-x86_64-with-centos-7.6.1810-Core
Python 3.6.8
Installer: RPM

azure-cli 2.25.0

Extensions:
image-copy-extension 0.2.8
```

**Additional context**

Microsoft case 2106020010003350 was opened for this issue. In the process of investigating that issue, we found that the connection reset behavior from the Azure side is intentional and needs to be handled by the Az CLI.

In particular, the blob will return connection reset when the file is written. We've noticed that this seems to happen in 32MB chunks, potentially due to upstream write buffering, hence the "large" requirement for replicating the issue.

**Suggested fix**

Microsoft proposed either using a lease, which is an exclusive write lock on the file or a snapshot. Write-locking a file used by the PaaS service seem like it would lead to unintended consequences on the back end, so I don't think this is a good plan.

Creating a snapshot, downloading the snapshot, then removing the snapshot seems like a very good plan for ensuring write consistency. This would be my suggested way of fixing/working around this bug.

**Workaround with curl**

A workaround is to use `az mysql server-logs list --resource-group $PAAS_HOST_RG --server $PAAS_HOST` to get a list of URLs with SAS for the logs. The appropriate SAS URL for the log in question can be fed to a very recent version of "curl" (newer than 7.52.0) which supports `--retry-connrefused` which will allow the download to complete. For details about curl retry, see [https://stackoverflow.com/questions/42873285/curl-retry-mechanism](https://stackoverflow.com/questions/42873285/curl-retry-mechanism)