BUG/MEDIUM: grpc: Fix several unaligned 32/64 bits accesses

BUG/MEDIUM: grpc: Fix several unaligned 32/64 bits accesses

There were several places in grpc and its dependency protobuf where unaligned
accesses were done. Read accesses to 32 (resp. 64) bits values should be performed
by read_u32() (resp. read_u64()).
Replace these unligned read accesses by correct calls to these functions.
Same fixes for doubles and floats.

Such unaligned read accesses could lead to crashes with bus errors on CPU
archictectures which do not fix them at run time.

This patch depends on this previous commit:
    861199fa71 MINOR: net_helper: Add support for floats/doubles.

Must be backported as far as 2.6.