Refactor S3Writer class to improve code structure

[jfzunigac]

S3Writer refactor changes included:

• Delete unused and redundant helper methods and class fields
• Introduce helper functions to reduce code duplicates
• Change File.renameTo() to Files.move() for buffer file renaming. The latter is much more reliable and offers better error handling
• Consolidate upload logic to one single method
• Add more comments and javadoc
• Add metric for number of messages written to buffer (so it can be compared with upstream logger metric)
• Introduce "logName" derived from singer log config name to use instead of logStream.getLogStreamName()