add precalculation interface

td-337: add error log when have to call pthread_cancel

Feature/query

fix TD-394

fix general/insert/insert_drop.sim error

Hotfix/sangshuduo/add case to test insert then import

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