MFV r274271:

Improve zdb -b performance:

• Reduce gethrtime() call to 1/100th of blkptr's;
• Skip manipulating the size-ordered tree;
• Issue more (10, previously 3) async reads;
• Use lighter weight testing in traverse_visitbp();

Illumos issue:

5243 zdb -b could be much faster

MFC after: 2 weeks