Various small C++17 code improvements.
Replace mapbox::variant with std::variant.
Remove the bt_u64 type wrapper; instead we know have `bt_value` which
wraps a variant holding both int64_t and uint64_t, and has contructors
to send signed/unsigned integer types into the appropriate one.
lokimq::get_int checks both as appropriate during extraction.
As a side effect this means we no longer do the uint64_t -> int64_t
conversion on the wire, ever, without needing the wrapper; although this
can break older versions sending large positive integers (i.e. larger
than int64_t max) those weren't actually working completely reliably
with mapbox variant anyway, and the one place using such a value in loki
core (in a checksum) is already fully upgraded across the network
(currently using bt_u64, but always sending a positive value on the
wire).
Jason Rhinelander
68c1899cda