FTL Shared Memory

[AzureMarker]

FTL is implementing a way for it and the API to share stats memory. This will eliminate the stats functions from the FTL socket API, and that processing can now happen in this API.

Each major FTL structure is in a separate shared memory mapping, and there is a string buffer map which holds the strings used.

The API will connect to these shared memory files and directly access the data to respond to API requests. There will be a way for the API to take a read lock on the structures, to avoid incorrect data.

Currently, the FTL code is in the new/shmem branch.

In a private prototype, I have found that the shmem crate is the best option for connecting to the shared memory. The shared_memory crate was a runner-up, but since we are not connecting to shared memory that it created, its locks and events are of no use, and it always opens the shared memory in read/write mode.