Parallel fast index creation

When doing fast index creation for multiple indexes InnoDB first reads the clustered index and creates temporary files containing unsorted index entries, one file per index. It then sorts those files one-by-one using the merge buffer (which is tunable in Percona Server) and another temporary file. It is possible to do that sorting in multiple threads. Naturally, with N threads it would require (N-1) more temporary files and (N-1) more merge buffers.

This is to implement an InnoDB alternative to myisam-repair-threads, i.e. multi-threaded fast index creation.