Cache all event offsets.

def _cache_offsets(self, up_to_index=None, verbose=True):
        """Cache all event offsets."""
        if not up_to_index:
            if verbose:
                self.print("Caching event file offsets, this may take a bit.")
            self.blob_file.seek(0, 0)
            self.event_offsets = []
            if not self.raw_header:
                self.event_offsets.append(0)
        else:
            self.blob_file.seek(self.event_offsets[-1], 0)
        for line in iter(self.blob_file.readline, ''):
            line = try_decode_string(line)
            if line.startswith('end_event:'):
                self._record_offset()
                if len(self.event_offsets) % 100 == 0:
                    if verbose:
                        print('.', end='')
                    sys.stdout.flush()
            if up_to_index and len(self.event_offsets) >= up_to_index + 1:
                return
        self.event_offsets.pop()    # get rid of the last entry
        if not up_to_index:
            self.whole_file_cached = True
        self.print("\n{0} events indexed.".format(len(self.event_offsets)))