Remove redundant crc32 calculation done in tfw_hash_str()

Previously, the algorithm calculated two parallel CRC32 values on the
same data to fill all the 64 bits of the output hash.

This change just removes the parallel calculation leaving zeros in the
high 32 bits of the thash.

That is done because:

1. The extra CRC32 brings no entropy to the hash value. The only
difference between low and high bytes of the hash is the 32 bits of
entropy stored in the initial state of the crc0 and crc1 variables.
From this point of view the additional crc32 is useless and redundant.

2. The extra CRC32 calculation makes the code a bit confusing and
slower.

3. Zeros in higher bits of the hash are beneficial for the HTrie
implementation because it reduces branching for higher-level nodes of
the tree (and thus improves caching).

Related to: #28 - replace hash function in tfw_sched_hash.c