Fix memory leak in task destruction (header not deallocated).

[vlovich]

What does this PR do?

This fixes the memory leak and makes it safe to create executors and destroy them without leaking memory.

Motivation

I figured I'd point Codex at the problem and it turns out the fix was straightforward.

Related issues

#448

Additional Notes

I verified this also in Valgrind.
valgrind --leak-check=full --trace-children=yes --suppressions=valgrind.supp target/debug/deps/glommio-d6605e578a7ecc14 executor::test::executor_shutdown_does_not_leak_eventfds

Before the fix it prints (twice because the test itself is within a spawned subprocess):

    ==3196982== LEAK SUMMARY:
    ==3196982==    definitely lost: 0 bytes in 0 blocks
    ==3196982==    indirectly lost: 0 bytes in 0 blocks
    ==3196982==      possibly lost: 28,476 bytes in 435 blocks
    ==3196982==    still reachable: 4,193 bytes in 40 blocks
    ==3196982==         suppressed: 560 bytes in 1 blocks
    ==3196982== Reachable blocks (those to which a pointer was found) are not shown.
    ==3196982== To see them, rerun with: --leak-check=full --show-leak-kinds=all
    ==3196982== 
    ==3196982== For lists of detected and suppressed errors, rerun with: -s
    ==3196982== ERROR SUMMARY: 12 errors from 12 contexts (suppressed: 1 from 1)

    ==3196979== 
    ==3196979== HEAP SUMMARY:
    ==3196979==     in use at exit: 866 bytes in 7 blocks
    ==3196979==   total heap usage: 1,058 allocs, 1,051 frees, 1,439,425 bytes allocated
    ==3196979== 
    ==3196979== LEAK SUMMARY:
    ==3196979==    definitely lost: 0 bytes in 0 blocks
    ==3196979==    indirectly lost: 0 bytes in 0 blocks
    ==3196979==      possibly lost: 0 bytes in 0 blocks
    ==3196979==    still reachable: 818 bytes in 6 blocks
    ==3196979==         suppressed: 48 bytes in 1 blocks

After the fix it's completely clean:

    ==3202589== Memcheck, a memory error detector
    ==3202589== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
    ==3202589== Using Valgrind-3.23.0.GIT and LibVEX; rerun with -h for copyright info
    ==3202589== Command: target/debug/deps/glommio-d6605e578a7ecc14 executor::test::executor_shutdown_does_not_leak_eventfds
    ==3202589== 

    running 1 test
    test executor::test::executor_shutdown_does_not_leak_eventfds ... ok

    test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 399 filtered out; finished in 3.35s

    ==3202589== 
    ==3202589== HEAP SUMMARY:
    ==3202589==     in use at exit: 592 bytes in 2 blocks
    ==3202589==   total heap usage: 1,043 allocs, 1,041 frees, 968,940 bytes allocated
    ==3202589== 
    ==3202589== LEAK SUMMARY:
    ==3202589==    definitely lost: 0 bytes in 0 blocks
    ==3202589==    indirectly lost: 0 bytes in 0 blocks
    ==3202589==      possibly lost: 0 bytes in 0 blocks
    ==3202589==    still reachable: 544 bytes in 1 blocks
    ==3202589==         suppressed: 48 bytes in 1 blocks
    ==3202589== Reachable blocks (those to which a pointer was found) are not shown.
    ==3202589== To see them, rerun with: --leak-check=full --show-leak-kinds=all
    ==3202589== 
    ==3202589== For lists of detected and suppressed errors, rerun with: -s
    ==3202589== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 1 from 1)

Checklist

[X] I have added unit tests to the code I am submitting
[X] My unit tests cover both failure and success scenarios
[X] If applicable, I have discussed my architecture