Remove memory leak introduced in #2

[wouterdb]

Hi all,

In #2 I introduced caches to improve performance. We have discovered that these caches introduce memory leaks.

Hereby a patch to fix this leak.

Details

• the functools.lru_cache locks all arguments and return value of the method it caches in memory
• this includes the 'self' reference.

We also filed a bug for this on python: https://bugs.python.org/issue44310

Backstory

• On devices with a large yang schema, we observed consistent growth of the memory footprint
• We spent several days searching for the cause, this was complicated by the complex interaction between lxml/libxml2 (which is written in C) and glibc (documented here, bottom of the page: http://xmlsoft.org/xmlmem.html) .
• We eventually discovered that the functools.lru_cache locks arguments in memory, using objgraph

[edvgui]

Hello @yuekyang ! Would it be possible to release this change? This is a pretty big deal and would help us a lot.

Thanks