Howard Hinnart's date.h is the library that was accepted as C++20
date/calendar support, so this is essentially a backport of C++20 date
time support.
(It does support timezone support, but requires more of the library and
that seems like overkill for what we need; this just prints UTC
timestamps instead, which need only a header-only include).
Step 1 of removing abseil from lokinet.
For the most part this is a drop-in replacement, but there are also a
few changes here to the JSONRPC layer that were needed to work around
current gcc 10 dev snapshot:
- JSONRPC returns a json now instead of an optional<json>. It doesn't
make any sense to have a json rpc call that just closes the connection
with returning anything. Invoked functions can return a null (default
constructed) result now if they don't have anything to return (such a
null value won't be added as "result").
