* WIP: start mongo engine
* WIP: mongo accessor and admin
* integrate mongo store to main
* disable mongo tests by default, only in CI
* connection with constructor
* add mongo buffered writer
* buffered mongo writer
* fix nil responses on an empty list from mongo
* missing mongo index for scores
* cancelable store
* add gridfs implementation of avatar store
* fix race on mongo session copy
* gridfs avatars without tmp files
* move avatar store
* minor comments and refactoring for avatar store
* merged from current master
* simplify gridfs reader
* lint: fix minor warns
* test mongo against env defined url
* pass MONGO_REMARK_TEST to docker and travis
* set dockerfile env for mongo test url
* increase connect timeout in mongo tests
* pass MONGO_REMARK_TEST to drone build
* add MONGO_REMARK_TEST to branch stage of drone
* mass mongo test url via build_args_from_env
* populate mongo IP to docker build hosts
* test env
* pass mongo ip via .mongo
* remove .mongo temp from git
* add .mongo -> env to linter step
* allow more time to autoflush writer test
* default mongo tests to "mongo" if not in env
* merge fresh master into
* add test for mongo cleanup
* msg for a failed test
* lazy fix for failed test
* add an ability to skip all mongo tests
* add backend dev instructions
* remove unused code from mongo server
* move mongo testing to connection_test
* restore testing.go
* lint: minor warns for testing code