Minor optimisation of XLogInsert()

Patch adds cacheline padding around parts of XLogCtl.

Idea from way back, but never got performance tested in a situation
where WALInsertLock was the bottleneck.

So this is speculative, in need of testing to investigate value.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services