vibed AssertError

[Arjan]

The application performs 1000+ requests to the same web api at 
max 10 requests per second.
Quite often it crashes producing the trace below.

There are 10 tasks each calling `requestHTTP( url, ...` in a 
loop, the loop body is guarantied never ready within a second.

Any ideas what might be causing this Assert to trigger?

The asserts checks are:
```
assert(!m_requesting, "Interleaved HTTP client requests 
detected!");
assert(!m_responding, "Interleaved HTTP client request/response 
detected!");
```

Stack trace:
```
[main(/cjf-fin) FATAL] TaskFiber getting terminated due to an 
uncaught core.exception.AssertError: 
core.exception.AssertError at C:\Users\arjan\AppData\Local\dub\packages\vibe-http\1.2.1\vibe-http\source\vibe\http\client.d(642): Interleaved HTTP client request/response detected!
[main(/cjf-fin) FATAL] ----------------
[main(/cjf-fin) FATAL] 0x00007FF7FB505F1A in d_assert_msg
[main(/cjf-fin) FATAL] 0x00007FF7FB3B4130 in 
vibe.http.client.HTTPClient.doRequest at 
C:\Users\arjan\AppData\Local\dub\packages\vibe-http\1.2.1\vibe-http\source\vibe\http\client.d(641)
[main(/cjf-fin) FATAL] 0x00007FF7FB3B1929 in 
vibe.http.client.HTTPClient.doRequestWithRetry at 
C:\Users\arjan\AppData\Local\dub\packages\vibe-http\1.2.1\vibe-http\source\vibe\http\client.d(631)
[main(/cjf-fin) FATAL] 0x00007FF7FB3AF8D8 in 
vibe.http.client.HTTPClient.request at 
C:\Users\arjan\AppData\Local\dub\packages\vibe-http\1.2.1\vibe-http\source\vibe\http\client.d(602)
[main(/cjf-fin) FATAL] 0x00007FF7FB3AF4AA in 
vibe.http.client.requestHTTP at 
C:\Users\arjan\AppData\Local\dub\packages\vibe-http\1.2.1\vibe-http\source\vibe\http\client.d(81)
[main(/cjf-fin) FATAL] 0x00007FF7FB3AF34E in 
vibe.http.client.requestHTTP at 
C:\Users\arjan\AppData\Local\dub\packages\vibe-http\1.2.1\vibe-http\source\vibe\http\client.d(75)
```