minor cleanup and updated comments

minor cleanup and updated comments

python tinode_grpc updated

readme updated

mongo is failing to compile

intra-cluster server messages have to be passed

Merge pull request tinode#713 from tinode/devel-18

Possible fix for a crash

another case of unfinished transaction

crash in store due to a bug in mysql adapter on missing auth record

crash in store due to a bug in mysql adapter on missing auth record