Previously, if a multipart parser reached an eof before the declared length, it
still let the writer wait and timeout (300s). This was especially noticed if
users toggled `serialize_worker_startup=true`. Then, the first worker would
buffer all the data (another undesirable behavior) until it reached a premature
eof and then get stuck. Other workers wouldn't start either and the request
would eventually crash.

Here we fix both issues:

 1) If we detect serialize_worker_startup=true then we switch back to the
 default parallel worker startup pattern. This how the MP parser was built to
 work. Otherwise it would buffer the whole attachment into memory until the
 first worker wasn't done and the others started. That defies the purpose of
 incremental attachment uploads. So to go with the grain of MP parser design we
 switch back to parallel worker startup.

 2) Let writers which wait on an EOF exit instead of deadlocking until a
 timeout. Writer waits for more bytes when the stream already reached EOF is
 because the user uploaded less than the declared number of bytes (attachment
 is too short). In that case we exit normally and let the parser monitors fire
 with `"attachment shorter than expected"` error.

Reproducer for the issue:

 * start dev/run cluster with 3 nodes
 * `s:multicall(fun() -> config:set("fabric", "serialize_worker_startup","true") end)`
 * curl -XPUT 'http://adm:[email protected]:15984/mptest'

Before PR:

```
curl  --max-time 10 -XPUT 'http://adm:[email protected]:15984/mptest/short' -H 'Content-Type: multipart/related;boundary="abc123"' --data-binary $'--abc123\r\nContent-Type: application/json\r\n\r\n{"_attachments":{"ohai":{"follows":true,"content_type":"text/plain","length": 4}}}\r\n--abc123\r\n\r\noha\r\n--abc123--'
curl: (28) Operation timed out after 10005 milliseconds with 0 bytes received
```

After PR
```
curl  --max-time 10 -X PUT 'http://adm:[email protected]:15984/mptest/short' -H 'Content-Type: multipart/related;boundary="abc123"' --data-binary $'--abc123\r\nContent-Type: application/json\r\n\r\n{"_attachments":{"ohai":{"follows":true,"content_type":"text/plain","length": 4}}}\r\n--abc123\r\n\r\noha\r\n--abc123--'
{"error":"bad_request","reason":"attachment shorter than expected"}
```