Pre-allocate space for slices.

Several of the messages store the parts that have a variable number of
elements as slices.  This commit modifies the code to choose sane defaults
for the backing arrays for the slices so when the entries are actually
appended, a lot of the overhead of growing the backing arrays and copying
the data multiple times is avoided.

Along the same lines, when decoding messages, the actual size is known and
now is pre-allocated instead of dynamically growing the backing array
thereby avoiding some overhead.