MEDIUM: cache: store sha1 for hashing the cache key

MEDIUM: cache: store sha1 for hashing the cache key

The cache was relying on the txn->uri for creating its key, which was a
big problem when there was no log activated.

This patch does a sha1 of the host + uri, and stores it in the txn.
When a object is stored, the eb32node uses the first 32 bits of the hash
as a key, and the whole hash is stored in the cache entry.

During a lookup, the truncated hash is used, and when it matches an
entry we check the real sha1.