use cpp-httplib for HTTP control plane (#1979)

refactored the old control plane code to use [cpp-httplib](https://github.com/yhirose/cpp-httplib) instead of a hand rolled HTTP server.  Makes the control plane code much more legible.  Also no longer randomly stops responding.

controller/DBMirrorSet.cpp

@@ -15,9 +15,12 @@
 
 namespace ZeroTier {
 
-DBMirrorSet::DBMirrorSet(DB::ChangeListener *listener) :
-	_listener(listener),
-	_running(true)
+DBMirrorSet::DBMirrorSet(DB::ChangeListener *listener)
+	: _listener(listener)
+	, _running(true)
+	, _syncCheckerThread()
+	, _dbs()
+	, _dbs_l()
 {
 	_syncCheckerThread = std::thread([this]() {
 		for(;;) {

controller/EmbeddedNetworkController.hpp

@@ -37,6 +37,8 @@
 
 #include <nlohmann/json.hpp>
 
+#include <cpp-httplib/httplib.h>
+
 #include "DB.hpp"
 #include "DBMirrorSet.hpp"
 
@@ -66,27 +68,9 @@ public:
 		const Identity &identity,
 		const Dictionary<ZT_NETWORKCONFIG_METADATA_DICT_CAPACITY> &metaData);
 
-	unsigned int handleControlPlaneHttpGET(
-		const std::vector<std::string> &path,
-		const std::map<std::string,std::string> &urlArgs,
-		const std::map<std::string,std::string> &headers,
-		const std::string &body,
-		std::string &responseBody,
-		std::string &responseContentType);
-	unsigned int handleControlPlaneHttpPOST(
-		const std::vector<std::string> &path,
-		const std::map<std::string,std::string> &urlArgs,
-		const std::map<std::string,std::string> &headers,
-		const std::string &body,
-		std::string &responseBody,
-		std::string &responseContentType);
-	unsigned int handleControlPlaneHttpDELETE(
-		const std::vector<std::string> &path,
-		const std::map<std::string,std::string> &urlArgs,
-		const std::map<std::string,std::string> &headers,
-		const std::string &body,
-		std::string &responseBody,
-		std::string &responseContentType);
+	void configureHTTPControlPlane(
+		httplib::Server &s,
+		const std::function<void(const httplib::Request&, httplib::Response&, std::string)>);
 
 	void handleRemoteTrace(const ZT_RemoteTrace &rt);
 
@@ -98,6 +82,8 @@ private:
 	void _request(uint64_t nwid,const InetAddress &fromAddr,uint64_t requestPacketId,const Identity &identity,const Dictionary<ZT_NETWORKCONFIG_METADATA_DICT_CAPACITY> &metaData);
 	void _startThreads();
 
+	std::string networkUpdateFromPostData(uint64_t networkID, const std::string &body);
+
 	struct _RQEntry
 	{
 		uint64_t nwid;