avoid bloat from CREATE INDEX CONCURRENTLY

Here's another small patch, this time from Simon Riggs. Maybe he already
posted it for this commitfest, but I didn't find it in a quick look so
here it is.

This patch reduces the amount of bloat you get from running CREATE INDEX
CONCURRENTLY by destroying the snapshot taken in the first phase, before
entering the second phase. This allows the global xmin to advance,
letting concurrent vacuum keep bloat in other tables in check.
Currently this implements the change for btree indexes only, but doing
it for other indexes should be a one-liner.

--
Álvaro Herrera