Replace single-use MPI datatypes with binary serialization

[fknorr]

So far, we are creating one custom composite MPI_Datatype for each MPI_Isend to transfer variably-sized commands and buffer subranges. For reasons not investigated further, this yields abysmal performance at least on IBM Spectrum MPI.

This PR replaces those single-use MPI datatypes with a serialization into frames, contiguous structs with a zero-sized array as the last member, a common idiom for typing a header together with variably-sized data. These are sent and received over MPI as simple byte streams. The smart pointer type unique_frame_ptr is introduced to allocate and manage frame types with arbitrary, variably-sized payloads.

As a downside, this change adds a semantically unnecessary allocation and copy of incoming buffer transfers: MPI receives data_frames, whereas the buffer manager keeps uncommitted transfers as raw_buffer_data. This might be noticeable for very large transfers. The solution is likely to get rid of the owning raw_buffer_data altogehter, but this will affect a lot of code, so it should be addressed in a follow-up PR.

[BlackMark29A]

To document the offline discussions we've had, and add additional comments:

[BlackMark29A]

LGTM.

[fknorr]

As per offline discussion yesterday, I've decided to duplicate the dependency count inside command_frame to allow using const command_frame & without its owning unique_frame_ptr. The previous approach with "mixed queries" to unique_frame_ptr and command_frame will become worse with #101 anyway, when command frames have to transport both dependencies and conflicts.

[psalz]

One minor thing, otherwise LGTM!