loci: use minimal allocation for fixed-length objects

Previously we allocated the full 64KB even for objects like the barrier reply,
which will only ever use 8 bytes. Allocating and zeroing unnecessarily large
wire buffers was a performance problem.

This change increases performance in my barrier microbenchmark by 46%.
Merge into master from pull request #237:
loci: use minimal allocation for fixed-length objects (https://github.com/floodlight/loxigen/pull/237)