memtx: add yields during index build

Memtx index build used to stall event loop for all the build period.
Add occasional yields so that the loop is not blocked for too long.
Also make index build set on_replace triggers so that concurrent
replaces are also correctly handled during the build.

Part of #3976

@TarantoolBot document
Title: memtx indices are now built in background

Memtx engine no longer stalls the event loop during an index build.
You may insert tuples into a space while an index build is in progress:
the tuples will be correctly added to the new index. If such tuple
violates the new indexes unique constraint or doesn't match the new
index format, the index build will be aborted.