use ABT after calling margo_init

[adammoody]

This removes the call to ABT_init and defers that task to Margo. Margo configures ABT in a particular way.

In particular, it increases the default ABT thread stack size from 4KB to 2MB:
https://github.com/mochi-hpc/mochi-margo/blob/ff094a5a26af19041860f9bba1e0e938c0194ae5/src/margo-init.c#L1533-L1534

which resolves some problems as described in:
pmodels/argobots#333

Alternatively, we could call ABT_init before margo_init. However, we likely would need to configure ABT to be compatible with what Margo expects. We need to set some ABT environment variables before calling ABT_init with something like:

    char env_str[64];
    sprintf(env_str, "%d", 2*1024*1024);
    setenv("ABT_THREAD_STACKSIZE", env_str, 0);

    ABT_init(argc, argv);

similar to what is done in margo here:
https://github.com/mochi-hpc/mochi-margo/blob/ff094a5a26af19041860f9bba1e0e938c0194ae5/src/margo-util.c#L29-L36

Description

Motivation and Context

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Testing (addition of new tests or update to current tests)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the UnifyFS code style requirements.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted.

[CamStan]

Looks good! Assuming @MichaelBrim doesn't have a reason we need to initialize Argobots first.

[adammoody]

Also, we should remember that if we're seeing strange segfaults in the future to try increasing ABT_THREAD_STACK_SIZE. Considering that, it's kind of a bummer that margo calls setenv with overwrite set to 1:

setenv("ABT_THREAD_STACKSIZE", env_str, 1);

https://github.com/mochi-hpc/mochi-margo/blob/ff094a5a26af19041860f9bba1e0e938c0194ae5/src/margo-util.c#L34

That means their setting will overwrite anything the user has set in their environment. Ugh.

I think they do read values from their config file, though:
https://github.com/mochi-hpc/mochi-margo/blob/ff094a5a26af19041860f9bba1e0e938c0194ae5/src/margo-init.c#L1528

I think environment variables are easier to work with if we have to change things. We could ask the margo team about switching the setenv to use overwrite=0.

[MichaelBrim]

At one time, there was a reason to call ABT_init() separately. I don't remember why now, and can't find a code path that should require that now, so this is OK by me.