Memoize elements in rewriter; use correct iteration

Memoize elements in rewriter; use correct iteration

Modified the logic of the :class:`.Rewriter` object such that it keeps a
memoization of which directives it has processed, so that it can ensure it
processes a particular directive only once, and additionally fixed
:class:`.Rewriter` so that it functions correctly for multiple-pass
autogenerate schemes, such as the one illustrated in the "multidb"
template.  By tracking which directives have been processed, a
multiple-pass scheme which calls upon the :class:`.Rewriter` multiple times
for the same structure as elements are added can work without running
duplicate operations on the same elements more than once.

Change-Id: I2cef13d51912f9d86ddd99b60e4d5b96dbf680ff
Fixes: #505