prevent Valkey crash when module is loaded twice

[dmitrypol]

addresses #372
I tested this on Valkey 8 and 9
@murphyjacob4 - I implemented the change per your recommendation.

[murphyjacob4]

For context - @dmitrypol and I debugged - the problem is when you load two instances of the module, they will actually read from the same symbols and cause initialization problems. It surfaces as a problem here because the static main thread variable must be unset (but it set by the other shared object).

[murphyjacob4]

Maybe it's worth having a "symbol collision" condition that we check prior to calling OnLoad? I guess it is nice if we can keep the main thread tracking before the OnLoad call, so that OnLoad will have it set.

So something like:

absl::Status verifyLoadedOnlyOnce() {
    static bool prev_loaded = false;
    if (prev_loaded) return absl::FailedPrecondition("The module is already loaded");
    prev_loaded = true;
    return absl::OkStatus();
}
...
  int ValkeyModule_OnLoad(ValkeyModuleCtx *ctx, ValkeyModuleString **argv,  \
                          int argc) {                                       \
    VMSDK_RETURN_IF_ERROR(verifyLoadedOnlyOnce()); \
    ... <existing code>

[allenss-amazon]

well, don't use absl::Status as the return type, use a boolean. Don't use VMSDK_RETURN_IF_ERROR.
Put a message in the log directly using a static string (no memory allocations ,please). Then return VALKEYMODULE_ERROR if it fails.