In this commit, we decrease the default fp rate to 19, or 1/2^19. We do
this as recent analysis by sipa on the bitcoin dev mailing list has
shown that optimally, we can use a value of 2^19 for the fp rate, while
use n=1.497137*2^P rather than n directly. As a result, we can shrink
the filter size by quite a bit, while still maintaining a sane false
positive value.
This change will reduce the total number of allocations required to
create/store the allocated filter as we’ll now perform a _single_
allocation, rather than one each time the the dynamically size slice
reaches capacity.
