Guarantee exclusive use of `HttpClientConnection`. (#273)

When calling the HTTP `send` / `open` functions, `acquireConnection` is
called to obtain a connection in state `Ready`. In the next code block,
the connection state is advanced to `RequestHeadersSending`. However,
returning from chronos `async` procs yields control, similar to `await`.
This means that a connection may be added to the pool in `Ready` state,
and then a different `acquireConnection` call may obtain a second ref.
Introducing a new `Acquired` state ensures consistency in this case.

No tests added due to this being scheduler dependent; ran manual tests
by adding a `doAssert item.state != HttpClientConnectionState.Acquired`
between `if not(isNil(conn)):` and `return conn`. Eventually, the assert
got hit after several hours of repeated tests, confirming the edge case
to be solved by applying this fix.

Not sure if it is by design that returning from an `async` proc yields.
Even if it's not, this should solve current HTTP issues in nimbus-eth2.

chronos/apps/http/httpclient.nim

@@ -38,6 +38,7 @@ type
     Resolving,                ## Resolving remote hostname
     Connecting,               ## Connecting to remote server
     Ready,                    ## Connected to remote server
+    Acquired,                 ## Connection is acquired for use
     RequestHeadersSending,    ## Sending request headers
     RequestHeadersSent,       ## Request headers has been sent
     RequestBodySending,       ## Sending request body
@@ -583,6 +584,7 @@ proc acquireConnection(session: HttpSessionRef,
         await session.connect(ha).wait(session.connectTimeout)
       except AsyncTimeoutError:
         raiseHttpConnectionError("Connection timed out")
+    res[].state = HttpClientConnectionState.Acquired
     session.connections.mgetOrPut(ha.id, default).add(res)
     return res
   else:
@@ -599,6 +601,7 @@ proc acquireConnection(session: HttpSessionRef,
         else:
           nil
     if not(isNil(conn)):
+      conn[].state = HttpClientConnectionState.Acquired
       return conn
     else:
       var default: seq[HttpClientConnectionRef]
@@ -607,6 +610,7 @@ proc acquireConnection(session: HttpSessionRef,
           await session.connect(ha).wait(session.connectTimeout)
         except AsyncTimeoutError:
           raiseHttpConnectionError("Connection timed out")
+      res[].state = HttpClientConnectionState.Acquired
       session.connections.mgetOrPut(ha.id, default).add(res)
       return res