Remove check for available streams

A connection should be pickable even if no streams are currently
available. In this case, when executing the query, the connection will
block reading from the streams channel [see func (c *Conn) exec(...)]
until one is available.

This avoids 'unavailable' errors when there are more than
numConns*numStreams queries pending.
Merge pull request #158 from miguelvps/stream_unavailable

Remove check for available streams