In online index creation, we set up a log for each index that is being
created. Other threads may modify the table while the index records
are being copied and sorted. Any thread that would modify the index
will write a log record that will be applied at the end of the index
creation.

Previously, we used to copy delete-marked records to the index, and
used to log operations with a higher granularity. This change gets rid
of the delete-marked records and the tracking of the delete-mark
flag. As a result, it decouples the purge of delete-marked records
from online secondary index creation. We simply log two types of
operations: ROW_OP_INSERT and ROW_OP_DELETE.

enum row_op: Make the definition private to row0log.cc.
Map the removed values as follows:
ROW_OP_DELETE_MARK: ROW_OP_DELETE (that is, purge delete-marked
records straight away).
ROW_OP_DELETE_UNMARK: ROW_OP_INSERT.
ROW_OP_PURGE: Remove (purge does not need to log anything).
ROW_OP_DELETE_PURGE: ROW_OP_DELETE (rollback of INSERT).

dict_index_online_log(): Remove. Declare row_log_online_op() in a wider scope.

row_log_apply_ops(): Do not operate on delete-marked records. Instead,
purge on delete and insert on delete-unmark.

rb:1390 approved by Jimmy Yang, Sunny Bains