BUG-1585412 Fixed Date parsing regex for MySQL 5.7

MySQL 5.7 general log dates are in RFC3339 format:
2006-01-02T15:04:05Z07:00
The previous version of the log parser was not able to parse
this format.

bin/pt-query-digest

@@ -9847,7 +9847,7 @@ sub new {
 
 my $genlog_line_1= qr{
    \A
-   (?:(\d{6}\s+\d{1,2}:\d\d:\d\d))? # Timestamp
+   (?:(\d{6}\s+\d{1,2}:\d\d:\d\d|\d{4}-\d{1,2}-\d{1,2}T\d\d:\d\d:\d\d\.\d+Z))? # Timestamp
    \s+
    (?:\s*(\d+))                     # Thread ID
    \s