Avoid deadlocks during insertion into SP-GiST indexes.

SP-GiST's original scheme for avoiding deadlocks during concurrent index
insertions doesn't work, as per report from Hailong Li, and there isn't any
evident way to make it work completely.  We could possibly lock individual
inner tuples instead of their whole pages, but preliminary experimentation
suggests that the performance penalty would be huge.  Instead, if we fail
to get a buffer lock while descending the tree, just restart the tree
descent altogether.  We keep the old tuple positioning rules, though, in
hopes of reducing the number of cases where this can happen.

Teodor Sigaev, somewhat edited by Tom Lane