volatile PointerType in alloc_fixed_pool.hpp

[mattpearson]

https://github.com/saleyn/utxx/blob/master/include/utxx/alloc_fixed_pool.hpp#L81

When m_next is declared as volatile and PointerType is offset_ptr, compiler complains:

error: no matching function for call to ‘boost::interprocess::offset_ptr::offset_ptr(const volatile pointer_type&)’

Removing the volatile modifier for m_next resolves the issue.

[saleyn]

I think the proper thing to do here is to migrate the pointers marked volatile to become std::atomic<pointer_type>. I wrote this code prior to porting the project to C++11 so this wasn't done before