fix: reduce ws meta tracking memory consumption

Fixes #1717.

Reduce memory consumption of string values used in tracking
what metadata we have sent for a ws connection by keeping
a cache of shared string values for this. This will deduplicate
the values and help with memory consumption with a large
number of AIS targets and multiple ws connections.

The cache is purged every 30 minutes to get rid of data
for AIS targets that have disappeared.

src/interfaces/ws.js

@@ -505,18 +505,39 @@ function processUpdates(app, pathSources, spark, msg) {
   })
 }
 
+/*
+  Keep a list of shared context-path strings that is shared across ws connections.
+  This way the string values are shared across ws connections and not recreated
+  for each context-path-ws combination. This reduces memory consumption for
+  multiple ws clients.
+  Nevertheless we need to purge this data eventually, otherwise the strings for 
+  AIS targets will stay forever, so implement a simple total purge. This may cause
+  some thrashing, but is better than not sharing the values.
+*/
+let canonical_meta_contextpath_values = {}
+const getContextPathMetaKey = (context, path ) => {
+  const contextPaths = canonical_meta_contextpath_values[context] || (canonical_meta_contextpath_values[context] = {})
+  const result = contextPaths[path] || (contextPaths[path] = `${context}.${path}`)
+  return result
+}
+setInterval(() => {
+  canonical_meta_contextpath_values = {}
+}, 30 * 60 * 1000)
+
 function handleValuesMeta(kp) {
-  if (kp.path && !this.spark.sentMetaData[this.context + '.' + kp.path]) {
+  const fullContextPathKey = getContextPathMetaKey(this.context, kp.path)
+  if (kp.path && !this.spark.sentMetaData[fullContextPathKey]) {
     const split = kp.path.split('.')
     for (let i = split.length; i > 1; i--) {
       const path = split.slice(0, i).join('.')
-      if (this.spark.sentMetaData[this.context + '.' + path]) {
+      const partialContextPathKey = getContextPathMetaKey(this.context, path)
+      if (this.spark.sentMetaData[partialContextPathKey]) {
         //stop backing up the path with first prefix that has already been handled
         break
       } else {
         //always set to true, even if there is no meta for the path
-        this.spark.sentMetaData[this.context + '.' + path] = true
-        let meta = getMetadata(this.context + '.' + path)
+        this.spark.sentMetaData[partialContextPathKey] = true
+        let meta = getMetadata(partialContextPathKey)
         if (meta) {
           this.spark.write({
             context: this.context,