- Added a new TableProperty, creation_time, to keep track of when the
  SST file is created.
- Creation_time:
  - On Flush: Set to the time of flush.
  - On Compaction: Set to the max creation_time of all the files
    involved in the compaction.
- Added a new TTL option to FIFO compaction options.

- Added unit tests.
- Split PickCompaction into PickTTLCompaction and PickSizeCompaction.
- Fixed a bug in BuildTable where time wasn't being passed to
  NewTableBuilder.

Also handled the return status of GetCurrentTime, so that a garbage
value is not used when return statu is not ok. time is set to 0 instead.

with a default value of 0.

Also moved GetTotalFilesSize into anonymouse namespace from
FIFOCompactionPicker class.

Since there is a potential chance that we could discard the files picked
in the main loop in PickTTLCompaction, move the log message to the right
place, to be right before creating the Compaction object.

Support FIFO-compaction-with-TTL only when max_open_files=-1, as the
creation_time embedded in every file's table properties need to be
consulted to figure out the files that need to be deleted. table_reader
embedded in a FileDescriptor could potentially get deleted if
max_open_files is not set to -1.

We could initialize the table_reader again to get the table properties
again, but it would involve a performance penalty due to reading new
blocks from disk. We could potentially support it in the future, but may
be not in the first version.

1. It is only supported with max_open_files = -1.
2. It is only supported with Block based table format.

Addressing review comments:
1. Pick only continguous files for deletion in FIFO-with-TTL.
2. Add a check to make sure that GetTableProperties() does not return a
null pointer, before accessing fields fruther.