MDEV-6258 MariaDB 10.0 performance schema timestamps relative to epoch

don't use the interval timer when absolute time values are needed

mysql-test/suite/perfschema/r/mks_timer-6258.result

+select distinct first_seen < DATE'2000-01-01', last_seen < DATE'2000-01-01' from performance_schema.events_statements_summary_by_digest;
+first_seen < DATE'2000-01-01'	last_seen < DATE'2000-01-01'
+0	0

mysql-test/suite/perfschema/t/mks_timer-6258.test

+--source include/not_embedded.inc
+#
+# MDEV-6258 MariaDB 10.0 performance schema timestamps relative to epoch
+#
+select distinct first_seen < DATE'2000-01-01', last_seen < DATE'2000-01-01' from performance_schema.events_statements_summary_by_digest;

storage/perfschema/pfs_digest.cc

@@ -207,7 +207,7 @@ find_or_create_digest(PFS_thread *thread,
   PFS_statements_digest_stat **entry;
   PFS_statements_digest_stat *pfs= NULL;
 
-  ulonglong now= microsecond_interval_timer();
+  ulonglong now= my_hrtime().val;
 
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