conditional shm get, eagain

Some time ago, an issue was raised: #1921, followed by a subsequent pull request: #1954.

The issue we aimed to address was selectively retrieving data from shared memory (shm) and deserializing it only if the version of the key in shm differed from what was stored in the least recently used (lru) cache or a local Lua variable. This feature could prove immensely useful for swiftly and accurately updating configuration changes while maintaining a cached copy in each worker for efficiency (which is validated upon request).

@agentzh, the original issue became somewhat complex due to the inclusion of both compare-or-get (cog) and compare-and-swap (cas) operations, as well as incorporating the original value into the get API. Could you or someone else reconsider this and possibly streamline it, for instance, into a function like get_conditional(key, flags?) returning value, flags, flags_match, where value and flags are returned only if flags_match is false?

Start by reading issues #1921 and pull request #1954 to understand the prior compare-or-get and compare-and-swap work. Then evaluate the proposed get_conditional(key, flags?) API and its shared-memory behavior; the issue is complete only when the streamlined return behavior and scope are defined and implemented.