no asynchronous shutdown method

[weissi]

async-http-client only has a synchronous shutdown method but it'd be great to have an asynchronous one too.

[weissi]

basically a non-sync prefixed version of

async-http-client/Sources/AsyncHTTPClient/HTTPClient.swift

Lines 74 to 86 in 51dc885

	public func syncShutdown() throws {
	switch self.eventLoopGroupProvider {
	case .shared:
	self.isShutdown.store(true)
	return
	case .createNew:
	if self.isShutdown.compareAndExchange(expected: false, desired: true) {
	try self.eventLoopGroup.syncShutdownGracefully()
	} else {
	throw HTTPClientError.alreadyShutdown
	}
	}
	}