More fine grained control of spin loops could be useful

In an attempt to address MDEV-34178, I enabled spin loops on dict_index_t::lock. This helps in some workloads and causes performance degradation in others, especially when there are large numbers of concurrent connections.

It could make sense to enable spin loops on a case-by-case basis, or to introduce a configuration parameter for choosing when index tree latch acquisition should include spin loops.