Adding DATE_8601 to the list of default Grok patterns

[berglh]

On line 65 of this pattern file is the following comment:

 # datestamp is YYYY/MM/DD-HH:MM:SS.UUUU (or something like it)

It turns out the format in this comment isn't actually covered with YYYY MM DD with any other delimiter other than hyphens such as in TIMESTAMP_ISO8601.

There are a lot of log files that contain YYYY/MM/DD, not just the official ISO8601 time stamp YYYY-?MM-?DD.

I made the DATE_8601 pattern and suffixed it to the DATE pattern so when DATESTAMP is use, it covers all three common use formats. The YYYY MM DD is in used in China, Korea, Taiwan, Hungary, Iran, Japan, Lithuania due to ISO 8601. Also plenty of other countries use this format. I think for completeness of the basic date patterns it has a place here with all three common delimiters.

I think perhaps DATE_8601 is the wrong name for this pattern. Using slashes or dot delimiters is not an official 8601 format according to ISO8601.

I'm suspecting we should change this, but I don't know what a suitable suffix would be as it's hard to aggregate the aforementioned countries into a region code like the other date patterns in the file.

[elasticsearch-release]

Jenkins standing by to test this. If you aren't a maintainer, you can ignore this comment. Someone with commit access, please review this and clear it for Jenkins to run; then say 'jenkins, test it'.

[tedder]

This would address #67 and #112. I'm not sure if it resolves it, as .. wouldn't the DATE_* entries match before the third option is hit? The sane way to do it would be to add a "mandatory 4-digit year" entry to pick it off first:

YEAR4 (?>\d{4})
DATE_8061 %{YEAR4}[./-]%{MONTHNUM}[./-]%{MONTHDAY}
DATE %{DATE_8061}|%{DATE_US}|%{DATE_EU}