HProfFilter Rewrite

[vjkoskela]

We should consider rewriting the parser using a well supported toolchain like Antlr (http://www.antlr.org/). However, we should research existing parser implementations for either the text or binary hprof encoding (e.g. https://github.com/eaftan/hprof-parser). In either case the parser must support multiple dumps to a single file, and unlike the current parser should preserve thread start/stop events.