Refactor query execution

[Preetam]

This giant function to execute queries isn't very clean.

Task: Design and implement abstractions for query execution.

Misc steps (subject to change):

• Create an aggregation interface. These interfaces should accept updates and eventually return some aggregate result. Example aggregations: sum, min, max, average, percentile, first, last, count, etc.
• Create an event iterator interface. This interface should accept a set of filters and return events that match that filter set.
• Create a filter interface. Can be done as part of Add more filter operators #84.