Feature/2.0tsdb

Hotfix/boundary

create/alter table: only send actual tag data to reduce message size

Feature/pytest output sql command to file

add test cases for filters

Feature/query

set up DB connection only when there is a continuous query

move note mention python2 is not supported by foundation earlier.

Feature/2.0tsdb

[TD-383]

change xenial to bionic for using python3.6 and fix pytest script.