Log.getOffsetsBefore(t, …) does not return the last confirmed offset before t

Let's say there are three log segments s1, s2, s3.

In Log.getoffsetsBefore(t, …), the offsetTimeArray will look like - [(s1.start, s1.lastModified), (s2.start, s2.lastModified), (s3.start, s3.lastModified), (logEndOffset, currentTimeMs)].

Let's say s2.lastModified < t < s3.lastModified. getOffsetsBefore(t, 1) will return Seq(s2.start).

However, we already know s3.firstAppendTime (s3.created in trunk). So, if s3.firstAppendTime < t < s3.lastModified, we should rather return s3.start.

This also resolves another bug wherein the log has only one segment and getOffsetsBefore() returns an empty Seq if the timestamp provided is less than the lastModified of the only segment. We should rather return the startOffset of the segment if the timestamp is greater than the firstAppendTime of the segment.