[6.0] Partially revert #7566 to fix amazon linux 2 crash

[bnbarham]

Reverts unifying main to be async.

Amazon Linux 2 has been crashing (see below) in almost every test run since this change. It appears the crash existed before it, it's just much more consistent now.

Not clear if this is the backtrace every time, it's almost always just getrn, but we have seen this once:

 0      0x0000ffffa4e4b948 getrn + 136 in libcrypto.so.1.0.2k
 1 [ra] 0x0000ffffa4e4bddc lh_delete + 55 in libcrypto.so.1.0.2k
 2 [ra] 0x0000ffffa4e4eaac int_thread_del_item + 123 in libcrypto.so.1.0.2k
 3 [ra] 0x0000ffffa4e4f564 ERR_error_string + 151 in libcrypto.so.1.0.2k
 4 [ra] 0x0000ffffa5316d68 Curl_close + 135 in libcurl.so.4.8.0
 5 [ra] 0x0000ffffa52c2374 Curl_conncache_close_all_connections + 371 in libcurl.so.4.8.0
 6 [ra] 0x0000ffffa52f8884 curl_multi_cleanup + 215 in libcurl.so.4.8.0
 7 [ra] 0x0000ffffa7b893b0 URLSession._MultiHandle.deinit + 255 in libFoundationNetworking.so
 8 [ra] 0x0000ffffa7b895ec URLSession._MultiHandle.__deallocating_deinit + 11 in libFoundationNetworking.so
 9 [ra] 0x0000ffffa8ce86f8 _swift_release_dealloc + 27 in libswiftCore.so
10 [ra] 0x0000ffffa8ce9290 bool swift::RefCounts<swift::RefCountBitsT<(swift::RefCountInlinedness)1> >::doDecrementSlow<(swift::PerformDeinit)1>(swift::RefCountBitsT<(swift::RefCountInlinedness)1>, unsigned int) + 207 in libswiftCore.so
11 [ra] 0x0000ffffa7b7d984 URLSession.deinit + 79 in libFoundationNetworking.so
12 [ra] 0x0000ffffa7b7d9d4 URLSession.__deallocating_deinit + 11 in libFoundationNetworking.so
13 [ra] 0x0000ffffa8ce86f8 _swift_release_dealloc + 27 in libswiftCore.so
14 [ra] 0x0000ffffa8ce9404 bool swift::RefCounts<swift::RefCountBitsT<(swift::RefCountInlinedness)1> >::doDecrementSideTable<(swift::PerformDeinit)1>(swift::RefCountBitsT<(swift::RefCountInlinedness)1>, unsigned int) + 351 in libswiftCore.so
15 [ra] 0x0000ffffa7b40f9c objectdestroy.11Tm + 23 in libFoundationNetworking.so
16 [ra] 0x0000ffffa8ce86f8 _swift_release_dealloc + 27 in libswiftCore.so
17 [ra] 0x0000ffffa8ce9290 bool swift::RefCounts<swift::RefCountBitsT<(swift::RefCountInlinedness)1> >::doDecrementSlow<(swift::PerformDeinit)1>(swift::RefCountBitsT<(swift::RefCountInlinedness)1>, unsigned int) + 207 in libswiftCore.so
18 [ra] 0x0000ffffa81c71f0 BlockOperation.__deallocating_deinit + 31 in libFoundation.so
19 [ra] 0x0000ffffa8ce86f8 _swift_release_dealloc + 27 in libswiftCore.so
20 [ra] 0x0000ffffa8ce9290 bool swift::RefCounts<swift::RefCountBitsT<(swift::RefCountInlinedness)1> >::doDecrementSlow<(swift::PerformDeinit)1>(swift::RefCountBitsT<(swift::RefCountInlinedness)1>, unsigned int) + 207 in libswiftCore.so
21 [ra] 0x0000ffffa7fb5b48 objectdestroy.20Tm + 23 in libFoundation.so
22 [ra] 0x0000ffffa8ce86f8 _swift_release_dealloc + 27 in libswiftCore.so
23 [ra] 0x0000ffffa8ce9290 bool swift::RefCounts<swift::RefCountBitsT<(swift::RefCountInlinedness)1> >::doDecrementSlow<(swift::PerformDeinit)1>(swift::RefCountBitsT<(swift::RefCountInlinedness)1>, unsigned int) + 207 in libswiftCore.so
24 [ra] 0x0000ffffa84f4e3c _Block_release + 271 in libBlocksRuntime.so
25 [ra] 0x0000ffffa84bedf0 __destroy_helper_block_8_32c35_ZTS29dispatch_block_private_data_s + 91 in libdispatch.so
26 [ra] 0x0000ffffa84f4e3c _Block_release + 271 in libBlocksRuntime.so
27 [ra] 0x0000ffffa84aa96c _dispatch_continuation_pop + 235 in libdispatch.so
28 [ra] 0x0000ffffa84aa79c _dispatch_async_redirect_invoke + 183 in libdispatch.so
29 [ra] 0x0000ffffa84b5c38 _dispatch_worker_thread + 467 in libdispatch.so
30 [ra] 0x0000ffffa78b4230 start_thread + 175 in libpthread-2.26.so

(cherry picked from commit e596d28)

[bnbarham]

@swift-ci please test