gcs: pre-allocate capacity of slice for uncompressed filter values
gcs: pre-allocate capacity of slice for uncompressed filter values

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.

dajohi/btcutil,dajohi/btcutil,btcsuite/btcutil,ppcsuite/ppcutil,btcsuite/btcutil,ppcsuite/ppcutil