Tags support (#280)

* Optimize tree merge

* Fix cache persistence

* Add docs for loader tool

* Remove unnecessary tree locks

* Update default loader config

* Add pyroQL query parser

* Implement basic query engine

* Refactor tests

* Extend storage Get to support queries

* Extend HTTP API to support query parameter

* Extend agent session to support tags

* Remove redundant url encoding

* Rename pypoql to flameql

* Extend agent CLI and config to support tags

* Add tag key validation

* Refine tag key naming

* Merge app name with tags

* Move tag merge logic to session

* Fix query result intersection

* Update script for docs gen

* Solve merge conflicts, refactor router

* Allow commas in tag values

* Remove unused code

* Move storage Key to segment package

This is necessary to avoid transitive import of logrus package in pkg/agent/profiler. In addition, the key itself is primarily used as a segment key, thus it should reside in the corresponding package.