SharedBuffer: Fix a denial of service

It's a very bad idea to increment the refcount on behalf of another
process. That process may (for either benign or evil reasons) not
reference the SharedBuffer, and then we'll be stuck with loads of
SharedBuffers until we OOM.

Instead, increment the refcount when the buffer is mapped. That way, a
buffer is only kept if *someone* has explicitly requested it via
get_shared_buffer.

Fixes #341

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