Pre-allocate space for high use slices.

This commit modifies the code to choose sane defaults for the backing
arrays for slices that involve a lot of appends such block locators, hash
processing, and needed transactions.  This is an optimization to avoid
the overhead of growing the backing arrays and copying the data multiple
times in the most common case.  This also prevents a leak in Go GC which
will likely ultimatley be fixed, but the efficiecy gains alone are worth
the change.