RPC: strictly require HTTP URI "/"

Previously, JSON-RPC clients accessed URI "/", and the JSON-RPC server
did not care about the URI at all, and would accept any URI as valid.

Change the JSON-RPC server to require URI "/" for all current accesses.

This changes enables the addition of future interfaces at different
URIs, such as pull request #1982 which demonstrates HTTP REST wallet
download.

Or, a future, breaking change in JSON-RPC interface could be introduced
by serving JSON-RPC calls from new URI "/v2/".
clear path cache after getting a proper config file (fixes #2605)

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
Merge pull request #2601 from jgarzik/uri-validate

RPC: strictly require HTTP URI "/"
Merge pull request #2608 from jonasschnelli/pathCacheFix

clear path cache after getting a proper config file (fixes #2605)