Change the way of working with shared memory

Change the way of working with shared memory as follows:

• split the shared segment into a fixed-size metadata area and a data area
• in the metadata area, store the key of the current data area
• when making changes to the data area, instead of changing in place, the writer should:

• allocate a new data area
• copy the information from the current data area
• make all necessary changes in the new area
• atomically write the key of the new data area to the metadata

Pros:

• at any given time, there is a consistent state by design
• there is no need for complex synchronization of readers and writers, a pointer to the data area can be stored in the normal address space of the process
• from the point above, it follows that no reader should wait for the completion of data changes
• a writer crash at any time will not cause data corruption, the key always points to completely correct data

Cons:

• it is necessary to think over the mechanism of deleting outdated segments
• similarly for new segments that were not "committed" as current due to a writer crash
• it is possible to worsen the performance of writing due to the need to copy data (it seems that this is not significant, but it should be measured)