Continuous memory increase in Sanic web server after calling `loop.start_tls(...)` in `connect_utils._create_ssl_connection`

@fantix arbeitet bereits daran.

Seit 17.2.2023.

* **asyncpg version**: `0.2.7`
* **PostgreSQL version**: `PostgreSQL 13.3 (Debian 13.3-1.pgdg110+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 10.2.1-6) 10.2.1 20210110, 64-bit`
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**: I use PostgreSQL installed in docker image `kartoza/postgis:pr-333-13-3.1`
* **Python version**: `3.8.10`
* **Platform**: `20.04.1-Ubuntu x86_64`
* **Do you use pgbouncer?**: Not sure, I use it indirectly. (behind `sqlalchemy`)
* **Did you install asyncpg with pip?**: Yes
* **If you built asyncpg locally, which version of Cython did you use?**: /
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: I see the code below is based on uvloop.

**Background**: I built a simple web server using `sqlalchemy` and `sanic`, then created a request handler function that can query my local PostgreSQL server running in docker. However, I saw continuous memory increase while benching my handler function.

After running `tracemalloc` for help, I got the result below:
```bash
Top 10 lines
#1: /home/tusimple/Workspace/projects/map-workdata-manager/venv/lib/python3.8/site-packages/asyncpg/connect_utils.py:700: 5670.1 KiB
new_tr = await loop.start_tls(
```

The code for creating the async_engine is:
```python
from sqlalchemy.ext.asyncio import create_async_engine
engine = create_async_engine("postgresql+asyncpg://{}:{}@{}/{}".format(...))
```

At first, I tried adding `?ssl=disable` after the previous connect string in order to prevent `connect_utils._create_ssl_connection` calls, and it works.
Then, I tried to make some stupid changes to the source code of `asyncpg`, because I think this SSL transport is not closed after creating new transport. (I'm not sure is that correct)
```python
# line 697
if hasattr(loop, 'start_tls'):
if do_ssl_upgrade:
try:
new_tr = await loop.start_tls(
tr, pr, ssl_context, server_hostname=host) # <--- here
except (Exception, asyncio.CancelledError):
tr.close()
raise
else:
new_tr = tr
# line 706
```
Finally, I modified the code starts from line 702 to:
```python
except (Exception, asyncio.CancelledError):
raise
finally:
tr.close()
```
After this modification, my web server keeps working fine, but I don't know whether this is a proper modification.

If you have any questions or ideas, please let me know, thank you so much!