Splits DialInfo.Timeout (defaults to 60s when using mgo.Dial()) into ReadTimeout
and WriteTimeout to address #160. Read/write timeout defaults to
DialInfo.Timeout to preserve existing behaviour.

Only used by tests, replaced by the pool-aware acquire socket functions:
	* AcquireSocketWithPoolTimeout
	* AcquireSocketWithBlocking

* `mongoCluster.syncServer()` no longer uses hard-coded 5 seconds
* `mongoCluster.isMaster()` no longer uses hard-coded 10 seconds

When unmarshalling serverTags, it is now an empty slice, instead of a nil slice.

`len(thing) == 0` works all the time, regardless.

Changes `DialWithInfo` to handle setting default values by setting the relevant
`DialInfo` field, rather than calling the respective methods in the hot path for:

	* `PoolLimit`
	* `ReadTimeout`
	* `WriteTimeout`