APIv2: Aggregates, timeseries, conversion_rate, hostname

[macobo]

Changes

• Aggregates now use new QueryBuilder
• Limited timeseries support in APIv2 - everything works, but results are only returned in bucket they're active in
• conversion_rate and group_conversion_rate metrics support
• Imports-related changes to support apiv2 - next PR will re-simplify
• Goals are preloaded into Query object for validation purposes
• event:goal breakdowns support. Now a single query and system is more flexible for renaming goals.
• event:page breakdows support. QueryOptimizer.split now splits query to two - sessions query uses visit:entry page
• fixed the memory leak around event:props:x creating symbols