Replace entire AsyncWebSocketBuffer infrastructure with std::shared_ptr

Based on commit 9172736 of dumbfixes
branch of 0xFEEDC0DE64 fork of ESPAsyncWebServer.

The entire purpose of having an AsyncWebSocketMessageBuffer is to
maintain a shared copy of a buffer sent to multiple clients until all of
them finish sending it. This can be replaced with a single message
object that contains a shared_ptr to the data being sent. For the single
client case, this is equivalent to non-shared buffer. For multiple
clients, the shared_ptr will maintain the buffer live until all messages
referencing it are destroyed. This simplifies the websocket
architecture.

Author: avillacis