dd-trace 0.6.0 on Node.js NPM

core: fix distributed tracing compatibility of generated IDs (#279)
core: fix already flushed traces being reused (#277)
core: fix client sampling resulting in incorrect metrics (#276)
express: fix repeated traces in some cases (#255)
http: fix repeated traces on Node <9 when tracing https requests (#268), thanks @alloy!

amqp10: add amqp10 integration (#254)
core: add support for sampling priority (#217), closes #208, thanks @stephenh and @alloy!
core: add an option to enable/disable the tracer (#209)
express: add option to record HTTP headers (#266), thanks @alloy!
graphql: add support for subscriptions (#249), fixes #248, thanks @clayne11!
graphql: add instrumentation for document parsing and validation (#245), thanks @alloy!
graphql: add option to record variables (#245), thanks @alloy!
hapi: add hapi integration (#282)
ioredis: add ioredis integration (#291), closes #259
koa: add koa integration (#288), closes #202
memcached: add memcached integration (#290), closes #163
pg: add support for pg 7.x (#250), thanks @archerabi!
restify: add restify integration (#289)

core: improve performance of generating IDs (#279)
deps: remove node-memwatch from optional dependencies (#281), closes #257
deps: bump url-parse version to address vulnerability (#265), closes #264, thanks @benjamine!