Enhance handling of large eventlogs

**Original reporter**: _jan.stolarek@_

When I enable detailed spark logging via -lf flag I end up with huge eventlog files (130MB). Attempting to load these into ThreadScope practically kills my OS - memory runs out, swapping begins and I am forced to kill TS (which takes some time before the OS actually responds and kills the process). This makes -lf flag useless for my program and I think this might not be uncommon situation. It would be good if TS supported some sort of lazy loading of big eventlogs, so users could at least view parts of the log.

The issue concerns ThreadScope loading large eventlog files produced with the -lf flag, but it does not identify the relevant files, tests, or loading entry point. Start by tracing how ThreadScope opens and retains eventlogs, then define how users should view part of a log without exhausting memory. Done means a 130MB eventlog can be opened and inspected without the current memory failure.

Written by the indexing model from the issue text.