add LZW compression to MongoObjectDatabase
added bulk putAll

Reduces import of wsa_rivers from 327 to 176 seconds
Added paralellization to mongo objectdb putAll

reduces import of wsa_rivers from 176 seconds to 73
Upgrade mongo-java-driver from 2.12.1 to 2.12.2
Merge remote-tracking branch 'groldan/mongo_improvements'
A couple fixes to JEGraphDatabase that prevented pushing from sparse clones.

map(ObjectId, ObjectId) was throwing an error if the "mapped" object id
didn't exist, instead of creating a new one.

ObjectIdBinding.entryToObject() was returning null instead of ObjectId.NULL
leading to an NPE's when NodeData.mappedTo was not expected to be null.
Merge branch 'master' of https://github.com/boundlessgeo/GeoGit
Fix bug in PushOp. It was updating local remote refs without the remotes prefix.