HA: discovery fixes

[mcspr]

• create object for each specialized discovery function to avoid accidentally sharing data (adjusted fix from Fix HA sensor discovery with MQTT #1957 @kiddouk )
• update mqtt discovery mechanism to enqueue topic+message pairs instead of directly sending them. most noticeable when using low-memory lwip or when just starting up the board, mqtt messages will fill the buffer and end up being lost. enqueue them manually ourselves and send one by one
• update mqttSendRaw to return whether sending succeeds (PID > 0)
• update mqttSendRaw to log only small messages, avoid breaking ws / telnet debug queues with ha discovery payloads
• small update to ws debug struct to also use std::pair<String, String>
• fix (again) discovery not sending on reconnection; send flag will now reset to "enabled" value
  resolves Espurna device unavailable in Homeassistant integration after restart from HS #1637

also supersedes #1370 (almost happy 1y anniversary)