NON-PRIMARY UNIQUE INDEX USING INNODB

This patch adds the HA_INPLACE_ADD_UNIQUE_INDEX_NO_WRITE
capability flag to InnoDB, indicating that concurrent reads
can be allowed while non-primary unique indexes are created.

This is an follow-up to Bug #11751388 which enabled concurrent
reads when creating non-primary non-unique indexes.

Test case added to innodb_mysql_sync.test.