Modify the process for seg.closed & Use of formalised uniform byte units & Modify the description of the openSegmentFile function

[NEMOlv]

Hi, this PR contains a three-part modification.

PART 1：Modify the Remove & Close function for segment.go I think the "seg.closed" for segment files should be set to "false" after calling "seg.fd.Close()". To avoid the case where closing the segment file fails, but the "seg.closed" is set to true.

PART 2：Use of formalised uniform byte units Just change “GB” to “1 * GB” when using byte constants. This has the advantage that byte constants can be used as units.

PART 3：Modify the description of the openSegmentFile function for segment.go From the wal.go, it looks like openSegmentFile is used to open both new and old segment file. Simply change "openSegmentFile a new segment file." to "openSegmentFile open a segment file."