Add Locker friend class to SBuf for protection against memory issues

When appending or otherwise modifying an SBuf based on a SBuf& or char*
the parameter used may be pointing at the MemBlob memory buffer
indirectly without holding a separate ref-count lock to it.

If 'this' SBuf then requires reallocation for any reason the char* or
buffer pointer taken from the SBuf&, which is being manipulated may in
fact be left pointing at invalid memory.

Utilize a private Locker class to create relatively cheap ref-count locks
on the store_ MemBlob when this problem MAY occur. This Locker needs to
be used on all non-const SBuf methods accepting char* or SBuf& argument.