ConnectionPool checkin should put connection back into the pool

[arthurnn]

Replaced #227 with this, so we PR against master.

We need to checkin the connection(put back in the pool) after use.
We cannot only depend on the reaper to get rid of connection, as threads could be sleeping or blocked by something else. If we make the connections rotate in the pool, we could have more threads using less connections.

just for an example, other gems are going the same thing:
https://github.com/rails/rails/blob/master/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb#L370
https://github.com/zk-ruby/zk/blob/master/lib/zk/pool.rb#L203
https://github.com/mperham/connection_pool/blob/master/lib/connection_pool.rb#L79

[fixes #223]

review @durran

[coveralls]

Coverage remained the same when pulling 6acc591 on conn_pool_checkin into 8433b87 on master.

[thibaudgg]

With this commit (using Moped master instead of beta3) I got a lot of Moped::Errors::AuthenticationFailure, running with Sidekiq and pool_size equals to concurrency (20).

Moped::Errors::AuthenticationFailure: The operation: #<Moped::Protocol::Commands::Authenticate @length=155 @request_id=1 @response_to=0 @op_code=2004 @flags=[] @full_collection_name="app13829477.$cmd" @skip=0 @limit=-1 @selector={:authenticate=>1, :user=>"heroku", :nonce=>"ec0e22bfbeb9454c", :key=>"20307c131d89639536c59da74595c481"} @fields=nil> failed with error 18: "auth fails" See https://github.com/mongodb/mongo/blob/master/docs/errors.md for details about this error.

[PROJECT_ROOT]/vendor/bundle/ruby/2.0.0/bundler/gems/moped-a87255b0dd17/lib/moped/authenticatable.rb:68:in `block in login`
[PROJECT_ROOT]/vendor/bundle/ruby/2.0.0/bundler/gems/moped-a87255b0dd17/lib/moped/node.rb:130:in `block in connection`
[PROJECT_ROOT]/vendor/bundle/ruby/2.0.0/bundler/gems/moped-a87255b0dd17/lib/moped/connection/pool.rb:159:in `with_connection`
[PROJECT_ROOT]/vendor/bundle/ruby/2.0.0/bundler/gems/moped-a87255b0dd17/lib/moped/node.rb:129:in `connection`
[PROJECT_ROOT]/vendor/bundle/ruby/2.0.0/bundler/gems/moped-a87255b0dd17/lib/moped/authenticatable.rb:65:in `login`
[PROJECT_ROOT]/vendor/bundle/ruby/2.0.0/bundler/gems/moped-a87255b0dd17/lib/moped/authenticatable.rb:28:in `block in apply_credentials`
[PROJECT_ROOT]/vendor/bundle/ruby/2.0.0/bundler/gems/moped-a87255b0dd17/lib/moped/authenticatable.rb:26:in `each`
[PROJECT_ROOT]/vendor/bundle/ruby/2.0.0/bundler/gems/moped-a87255b0dd17/lib/moped/authenticatable.rb:26:in `apply_credentials`
[PROJECT_ROOT]/vendor/bundle/ruby/2.0.0/bundler/gems/moped-a87255b0dd17/lib/moped/cluster.rb:227:in `block in with_primary`
[PROJECT_ROOT]/vendor/bundle/ruby/2.0.0/bundler/gems/moped-a87255b0dd17/lib/moped/node.rb:212:in `block in ensure_primary`
[PROJECT_ROOT]/vendor/bundle/ruby/2.0.0/bundler/gems/moped-a87255b0dd17/lib/moped/executable.rb:25:in `execute`
[PROJECT_ROOT]/vendor/bundle/ruby/2.0.0/bundler/gems/moped-a87255b0dd17/lib/moped/node.rb:211:in `ensure_primary`
[PROJECT_ROOT]/vendor/bundle/ruby/2.0.0/bundler/gems/moped-a87255b0dd17/lib/moped/cluster.rb:226:in `with_primary`
[PROJECT_ROOT]/vendor/bundle/ruby/2.0.0/bundler/gems/moped-a87255b0dd17/lib/moped/query.rb:379:in `update`
[PROJECT_ROOT]/vendor/bundle/ruby/2.0.0/bundler/gems/moped-a87255b0dd17/lib/moped/query.rb:420:in `upsert`