add TsdbClient and enable http chunk encoding

1. add TsdbClient to handle interaction with opentsdb table.
2. Application now supports a new type of query: chunked query
3. chunked query will push data points from web server to front end
   immediately once it get points from opentsdb table
4. add TConstants to hold all TsdbClient constants
5. add DashHelper to provide utility method to TsdbClient