Remove usage of NetworkCache

slapcache was using a old class NetworkCache to access shacache. Remove it in favor of standard NetworkcacheClient.

Please note that slapcache is still useful for now because slapos.libnetworkcache has no way to access a key without a MD5.

So basically, right now slapcache is:

• defining a new entry key in the section [shacache] in the config file of libnetworkcache
• retrieve all documents matching this key in shacache and select the one with newest timestamp