fix: do not apply timeout errors on inactive conns

when selector emits a timeout error, the full list of connections in the
pool is traversed. This is in itself not very performant, but the
problem is that the accounting is also done with connection which
weren't selected, such as the inactive connections. So we skip this for
them for now.

lib/httpx/connection.rb

@@ -554,6 +554,10 @@ module HTTPX
           ex.set_backtrace(error.backtrace)
           error = ex
         else
+          # inactive connections do not contribute to the select loop, therefore
+          # they should fail due to such errors.
+          return if @state == :inactive
+
           if @timeout
             @timeout -= error.timeout
             return unless @timeout <= 0