Replace homebrewed smart pointer with std::shared_ptr and std::weak_ptr

Based on commit 8483354 of dumbfixes
branch of 0xFEEDC0DE64 fork of ESPAsyncWebServer.

Buffers to data that is sent to all clients at once are stored as a
single copy of the data with multiple references to it. This usage is
one for which std::shared_ptr should be used. Each client having an
active std::shared_ptr to a master buffer keeps the buffer alive. Once
all clients have finished using their reference, the std::weak_ptr
ensures that the AsyncWebSocketMessageBuffer goes out of scope
and gets invalidated. This invalidation is checked with the lock()
method of the std::weak_ptr.

Also some code formatting while at it.

A future commit will introduce an unification between single-client
buffers and multiple-client buffers in such a way that the master buffer
list in the AsyncWebSocket class will be redundant and will be removed.
Therefore this stage is only temporary.

Author: avillacis