amari.drb: Rework _BitSync to be multicell-aware

To be able to handle multicell configurations rework _BitSync to handle
UEs that might be associated with multiple cells at the same time, e.g.
in the case of Carrier Aggregation.

For this BitSync interface is reworked correspondingly, then it uses
tx_bytes total->percell splitter, we added in the previous patch, then
it uses BitSync1 helpers, which each work on the stream of one
particular cell, and then the result is combined back into multicell _Utx.

The internals of new BitSync1 are very similar to original old BitSync implementation.
The new BitSync, that wraps many BitSync1s, is new.

The Sampler still accepts UEs with one cell only.
In the next patch we will update the Sampler to handle multicell
configurations as well.