Abstract over the way eventlogs are flushed

Currently eventlog data is always written to a file `progname.eventlog`.
This patch introduces the `flushEventLog` field in `RtsConfig` which
allows to customize the writing of eventlog data.

One possible scenario is the ongoing live-profile-monitor effort by
@NCrashed which slurps all eventlog data through `fluchEventLog`.

`flushEventLog` takes a buffer with eventlog data and its size and
returns `false` (0) in case eventlog data could not be procesed.

Reviewers: simonmar, austin, erikd, bgamari

Reviewed By: simonmar, bgamari

Subscribers: qnikst, thomie, NCrashed

Differential Revision: https://phabricator.haskell.org/D2934