- use last minor version for each major serie of
    mongodb
  - use travis 'go' param instead of eval "$(gimme $GO)"
    as it fails to install correctly last minor version
    ( 1.8.x notation for example)

also re-enable TestFindIterSnapshot as it was fixed
a long time ago

fix TestAuthX509CredRDNConstruction test: need to
create an user with {"username": subject} before
trying to login

Make sure that "rs3/127.0.0.1/40031" is elected
at primary.
Create user before running 'addShard' command as
it requires root access
Also add a retry mechanism to make sure that shard
are correctly added
cf https://docs.mongodb.com/manual/tutorial/deploy-shard-cluster/

require maxWireVersion >= 6 on server side, and
`experimental=opmsg` in the connection string

  - get `MaxMessageSizeBytes` and `MaxWriteBatchSize`
    from handshake
  - allow unacknowledged writes with `moreToCome` flag
  - split bulk operations in batch of `maxWriteBatchSize`
  - add 'experimental' param in URL. To enable an experimental
    feature, add `experimental=featureName` in the connection
    URL

flush logout directly

Previously, `flushLogout()` was called at the beginning
of each Query to the database.
To avoid these useless calls, we flush logout directly
when `Logout()` or `LogoutAll()` is called

SERVER-31049 is fixed in 3.4.10, so re-enable
it

use the same pool for send and received
messages.
Slices are returned to the pool without being
resized.

Default allocation size might need to be updated
(currently 256, no benchmarks available yet)