Although it is possible to get the command name for each msg type by
creating an instances of the type and calling the Command method against
it, it's slightly more efficient to simply allows callers to have direct
access to the exported constants.
This is currently really useful for the reject message since callers need
to be able to examine the command type to determine whether or not the
hash field needs to be included.
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.
