simpleperf: merge records from different buffers in memory.

simpleperf: merge records from different buffers in memory.

By reading records from all buffers at once, we can merge records
in memory instead of sorting them in perf.data. To make it clear,
this patch only contains the code to merge records in memory, and
I will remove old method later.

Bug: http://b/32343227
Test: run simpleperf_unit_test.
Test: run simpleperf_runtest.py.
Change-Id: Iea2da06c072243c2014f43c8aa6d96a23cfb9123