Optimization: delay clearing of page_full flag until page have at least 25% of free space. It saves few costly mark_full() calls when space is actively allocated and reclaimed in highly concurrent environment and reduces PP contention.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@57811 65644016-39b1-43b1-bf79-96bc8fe82c15