mysql,mssql: improve $__timeFilter, $__timeFrom, $__timeTo macros
The $__timeFilter macro now uses a BETWEEN with timestamps instead of
comparing epochs
The $__timeFrom and $__timeTo macros now return strings of format
YYYY-MM-DDThh:mm:ssZ instead of epoch
Removes the macro function . Macro functions should not do UTC/timezone conversion - they should
work in the same way as postgres and mysql datasource implementations.
Grafana and Microsft SQL Server should be run on servers with UTC timezones.
Earlier the division of interval was done using whole numbers resulting in that important information
was lost/too many time series merged to the same group. Now using division of floating point and rounding
up to solve the problem
