In an I/O bound concurrent INSERT test conducted by Mark Callaghan,
spin loops on dict_index_t::lock turn out to be beneficial.

This is a mixed bag; enabling the spin loops will improve throughput
and latency on some workloads and degrade in others.

Reviewed by: Debarun Banerjee
Tested by: Matthias Leich
Performance tested by: Axel Schwenke