Merge branch 'ta/config-set'

Add in-core caching layer to let us avoid reading the same
configuration files number of times.

* ta/config-set:
test-config: add tests for the config_set API
add `config_set` API for caching config-like files

diff --cc Documentation/technical/api-config.txt
Simple merge

diff --cc Makefile
Simple merge

diff --cc cache.h
Simple merge

diff --cc config.c
Simple merge