[TS-864] Buffer requests when there aren't enough sessions

A new module elysium_bs_serial.erl is introduced (buffering strategy serial). It queues requests when thre are too many by placing them on an ets_buffer FIFO queue independent of the open session queue. This module is a pure functional interface with message passing and spawned functions to collect replies. In future it will become an implementation of a behaviour so that application developers can add other strategies besides a straight serial buffer.