Added SerializerTest.AddDeleteRepeatedly test with some debugfs.
Added SerializerTest.AddDeleteRepeatedlyWithIndex, removed debugfs, reproduced #1691.
Removed a debugf in run_AddDeleteRepeatedly.
Added comments about the goals of run_AddDeleteRepeatedly with regard to what issue it is testing.
Reduced number of iterations in run_AddDeleteRepeatedly to 2000.
Made the data block manager avoid putting empty extents into the young extent queue.
Merge branch 'v1.11.x' into sam_1691
Merge branch 'v1.11.x' into sam_1691
Made compile with SEMANTIC_SERIALIZER_CHECK=1.

Because log_serializer_t had a "using serializer_t::make_io_account;"
line.  Made semantic_checking_serializer_t have that line too, made
log_serializer_t have it only when SEMANTIC_SERIALIZER_CHECK is not
defined.