Move spinlock statistics dump code out of the connection statistics dump
function, we don't want to dump spinlock information on every source we
list.

Use local WT_CONNECTION_IMPL variable instead of S2C.
Output a line even when no spinlocks were blocked, that way the logging
statistics will have roughly the same timeline as other statistics (I
say "roughly", because if the spinlock hasn't yet registered, the
timeline won't match exactly because unregistered spinlocks don't output
anything until they're registered, but over a reasonably long run they'll
be close).

Don't clear the blocked count until configured to do so.
Don't create a report unless both spinlocks are initialized.