fix: set max_heap_size to RealtimeChannel the same as the websocket transport

[edgurgel]

Changes:

• Set max_heap_size the same as the websocket transport
• Set fullsweep_after the same as the websocket transport

From Erlang docs:

A few cases when it can be useful to change fullsweep_after:

• If binaries that are no longer used are to be thrown away as soon as possible. (Set Number to zero.)

• A process that mostly have short-lived data is fullsweeped seldom or never, that is, the old heap contains mostly garbage. To ensure a fullsweep occasionally, set Number to a suitable value, such as 10 or 20.

• In embedded systems with a limited amount of RAM and no virtual memory, you might want to preserve memory by setting Number to zero. (The value can be set globally, see erlang:system_flag/2.)

RealtimeChannel and the websocket transport match the second bullet point as the data is usually sent and received but not kept around.

[coveralls]

coverage: 85.507% (+0.2%) from 85.287%
when pulling 339c20d on fix/max-heap-size-part-2
into d5658ad on main.