database: when querying, move latency counter instead of copying
database: when querying, move latency counter instead of copying

It is comprised of two time points. Let's move it instead of copying it.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>
Message-Id: <c7c155c77780e188bfbe05881c81ce86456016d5.1478111467.git.7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

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