Switch from adaptive pthread mutexes by default, based on:

http://smalldatum.blogspot.com/2014/08/the-innodb-mutex.html

There's more information here:

http://stackoverflow.com/questions/19863734/what-is-pthread-mutex-adaptive-np

Add a new option, --pthread_adaptive to configure adaptive mutexes.

Don't list --pthread-logging in the --with-spinlock=XXX configure usage
message, that's not for public use.

Add a tuning page that talks about mutex choice.