Use move in WeakRingBuffer where capable. Allow one-sized ring buffers

[Hattonuri]

By this time we are not allowed to create one-sized channel just because of asserts

i suppose this is because of:

libcds/cds/opt/buffer.h

Line 343 in 9985d2a

assert( !c_bExp2 || (m_nCapacity & (m_nCapacity - 1)) == 0 );

but 1 & (0) == 0 . so its a valid power of two

[TomlyYang]

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