JsonRpcConnection#Send*() queues messages in memory even after #Disconnect()

[Al2Klimov]

Describe the bug

1. JsonRpcConnection#Send*() posts a closure with the message in question into its strand which processes such sequentially
2. That closure inserts the message into #m_OutgoingMessagesQueue

Especially ApiListener#ReplayLog() fills it aggressively. After some time that queue is emptied by #WriteOutgoingMessages(). Unless it's been stopped, either by itself or by #Disconnect().

Expected behavior

• #Disconnect() immediately sets an atomic bool
• (1) and (2) from above happen only depending on that bool (also consult https://en.cppreference.com/w/cpp/atomic/memory_order)
• The closure #Disconnect() posts clears #m_OutgoingMessagesQueue ASAP

Additional context

ref/IP/48306
ref/IP/52366

[Al2Klimov]

@lippserd Admittedly this was already present in v2.11.x with the new network stack, same for what actually fills our queues:

• Start ApiListener#SyncClient() in the thread pool #8475

[Al2Klimov]

Btw. also an unnecessary in-memory queue:

• Drop ApiListener#m_RelayQueue #9855