Make event queue size adjustable via Kconfig

[mkfrey]

Fixes #121

[nietaki]

Can confirm, this helps in my case - changing the queue size to to 512 made the difference between the server sometimes dying while serving on 200kB files to successfully serving >1MB files with no faults.

In case someone else reading is using platformio, here's how to use the code in this PR before it's merged in:

platformio.ini:

lib_deps = 
  https://github.com/mkfrey/AsyncTCP#kconfig_queue_size
  ESP Async WebServer

build_flags = -DCONFIG_ASYNC_TCP_EVENT_QUEUE_SIZE=512