[SPARK-2845] Add timestamps to block manager events.

These are not used by the UI but are useful when analysing the
logs from a spark job.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #654 from vanzin/bm-event-tstamp and squashes the following commits:

d5d6e66 [Marcelo Vanzin] Fix tests.
ec06218 [Marcelo Vanzin] Review feedback.
f134dbc [Marcelo Vanzin] Merge branch 'master' into bm-event-tstamp
b495b7c [Marcelo Vanzin] Merge branch 'master' into bm-event-tstamp
7d2fe9e [Marcelo Vanzin] Review feedback.
d6f381c [Marcelo Vanzin] Update tests added after patch was created.
45e3bf8 [Marcelo Vanzin] Fix unit test after merge.
b37a10f [Marcelo Vanzin] Use === in test assertions.
ef72824 [Marcelo Vanzin] Handle backwards compatibility with 1.0.0.
aca1151 [Marcelo Vanzin] Fix unit test to check new fields.
efdda8e [Marcelo Vanzin] Add timestamps to block manager events.