Fix failing requests when ibrowse_lb process does not stop gracefully.

This behavior can be simulated in the current codebase by calling
```erlang
ibrowse_lb:stop(Pid)
```
where `Pid` is one of the currently running `ibrowse_lb` processes.  If the process is stopped at the right time, running `ets:tab2list(ibrowse_lb)` will confirm that the process record is still in the table, which would make `ibrowse`send `spawn_connection` messages to a process that does not exist.

`ibrowse` does not recover from the condition automatically.