Encapsulation layer encap.c: Reentrancy and Robustness Analysis and Rework

There is number of problems with the encapsulation layer:

Concurrency: TCP responses can be sent from multiple tasks concurrently, which is not properly respected by the source code (allocation of send buffers)

Delay: When sends at the lwIP fail due to no send buffer being available, retries will be performed in a 10ms raster, which imposes a certain delay on the TCP stream

It may be a good idea to introduce a job for sending, leading to strict serialization of events, thus keeping the number of required lock-operations low.

It is also required to split that module into multiple files, think about certain function and variable names and harden it a little.