The new variable is updated to contain the sum of the size of
PerHostRequestQueue::mQueuedRequests of every PerHostRequestQueue
object, and thus the total number of queued requests for all hosts
together.
Also added PerHostRequestQueue::host_queued_plus_added_size() which
returns the the sum of queued requests plus the requests already added
to the multi handle for this particular hostname.
Adds a std::map for hostname (or urls) --> PerHostRequestQueue
objects. The latter keeps track of the number of added curl easy
requests and decides if a new request should be throttled or
not, as well as provides the queue to queue throttled requests.
At the moment CurlConcurrentConnectionsPerHost is set to 16,
because things really don't work without LL supporting connection
reuse if we limit it to 2. CurlConcurrentConnectionsPerHost is
also set to non-persistent so that we can easily change it in the future
(once we decide on it's final value it can be set to persistent).
