support read preferences

[linearb]

txmongo should, like pymongo, fully support read preferences, to permit, for example, failover
to primary, or nearest. Behavior, should be like pymongo 3.2.2 as described here:
https://api.mongodb.org/python/current/api/pymongo/read_preferences.html#module-pymongo.read_preferences.

PRIMARY: Read from the primary. This is the default read preference, and provides the strongest consistency. If no primary is available, raise AutoReconnect.
PRIMARY_PREFERRED: Read from the primary if available, otherwise read from a secondary.
SECONDARY: Read from a secondary. If no matching secondary is available, raise AutoReconnect.
SECONDARY_PREFERRED: Read from a secondary if available, otherwise from the primary.
NEAREST: Read from any available member.

(Note an edge case that should be handled correctly:
pymongo, when connected to a mongod that happens to be a member of a replica set,
using SECONDARY and the replicaSet option unset, does not failover to other replicaset members.
But the current txmongo code seems to append the members of the set to __allnodes whether or
not replicaSet is set. )

[E4gle]

Are there any updates regarding this request ?
It would be really useful if this was supported, without this feature using replicaSet is pointless since all connections are always directed to the primary.

[psi29a]

If someone would like to get their hands dirty and contribute a PR, we'd be happy to review. :)