Improve gist XLOG code to follow the coding rules needed to prevent

Improve gist XLOG code to follow the coding rules needed to prevent
torn-page problems.  This introduces some issues of its own, mainly
that there are now some critical sections of unreasonably broad scope,
but it's a step forward anyway.  Further cleanup will require some
code refactoring that I'd prefer to get Oleg and Teodor involved in.