watcher silently fails when receiving huge data

Bug detected while testing octavia/taskflow with etcd3gw.

When using a watcher, if a huge payload is inserted in etcd, the watcher reads it and then fails silently, it no longer receives any data after that.

Reproducer:

======
from concurrent.futures import ThreadPoolExecutor
import time

from etcd3gw.client import Etcd3Client

def watcher_func(client):
    watcher, watcher_cancel = client.watch_prefix("/myprefix")
    for event in watcher:
        print(event)

with ThreadPoolExecutor(max_workers=1) as executor:
    client = Etcd3Client(host='192.168.1.15', port=2379)
    future = executor.submit(watcher_func, client)

    client.put("/myprefix/foo", 'a' * 100)
    time.sleep(1)
    client.put("/myprefix/foo", 'a' * 10000)
    time.sleep(1)
    client.put("/myprefix/foo", 'b')
======

when executing the reproducer print(event) displays only the 2 first events.

The issue is in the _watch function (https://github.com/openstack/etcd3gw/blob/master/etcd3gw/watch.py#L22-L24), in case of big payload, the data of the event is returned in 2 chunks: the first one contains the json doc, and the second only a '\n'

then json.loads("\n") triggers an exception

When adding a try/except block:

Traceback (most recent call last):
  File "/home/gthiemon/venv/lib64/python3.9/site-packages/etcd3gw/watch.py", line 25, in _watch
    payload = json.loads(decoded_line)
  File "/usr/lib64/python3.9/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python3.9/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python3.9/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 2 column 1 (char 1)