circular_buffer: make move-constructor noexcept
circular_buffer: make move-constructor noexcept

Which it is, since the only data member is a trivially-constructible
struct. This enables embeddeding circular_buffers without it
falling-back to copy its elements on expand().

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <5d214ba72fad1fb3b935440694670b5f6dbad09d.1513322013.git.a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

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