WasmFS: Reading from fetch backend in a thread doesn't work

Fetch backend runs in a separate thread. So when reading from another thread it will try to communicate with backend thread. But because the communication is done through the main thread, if main is busy, other threads can’t communicate. The program just hangs. Not sure if this can be considered as a bug.

**Version of emscripten/emsdk:**

```
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.23-git (84ad56fdb8acbaf07178998bdbb412ecb70e8267)
clang version 16.0.0 (https://github.com/llvm/llvm-project 9879261d7a061cc1df18f6fe164f42044fb4cff1)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: D:\emsdk\3.1.20\upstream\bin
```

**Failing command line in full:**

```
#include
#include
#include
#include
#include
#include
#include
#include

int main()
{
// File contents - "hello"
const char *filePath = "/file.txt";
const char *url = "http://localhost:8000/file.txt";

backend_t backend = wasmfs_create_fetch_backend(url);
int fd = wasmfs_create_file(filePath, S_IRWXU | S_IRWXG | S_IRWXO, backend);
assert(fd);

std::thread t ([=]()
{
std::ifstream input(filePath);
std::string text;
input >> text;
std::cout << text << std::endl;
});

t.join();

assert(close(fd) == 0);
assert(std::filesystem::remove(filePath));

std::cout << "Finished!" << std::endl;

return 0;
}
```

**Full link command and output with `-v` appended:**

```
D:\emscripten>em++ -v -std=c++17 D:/build/test.cpp -o D:/build/test.html -s EXIT_RUNTIME=1 -s WASMFS=1 -s USE_PTHREADS=1 -sPTHREAD_POOL_SIZE=2
"D:/emsdk/3.1.20/upstream/bin\clang.exe" --version
"D:/emsdk/3.1.20/upstream/bin\clang++.exe" -target wasm32-unknown-emscripten -fignore-exceptions -fvisibility=default -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -D__EMSCRIPTEN_SHARED_MEMORY__=1 -DEMSCRIPTEN -ID:\emscripten\cache\sysroot\include\SDL --sysroot=D:\emscripten\cache\sysroot -Xclang -iwithsysroot/include\compat -v -std=c++17 -pthread D:/build/test.cpp -c -o C:\Users\kdots\AppData\Local\Temp\emscripten_temp_rxxb846o\test_0.o
clang version 16.0.0 (https://github.com/llvm/llvm-project 9879261d7a061cc1df18f6fe164f42044fb4cff1)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: D:\emsdk\3.1.20\upstream\bin
(in-process)
"D:\\emsdk\\3.1.20\\upstream\\bin\\clang++.exe" -cc1 -triple wasm32-unknown-emscripten -emit-obj -mrelax-all --mrelax-relocations -disable-free -clear-ast-before-backend -main-file-name test.cpp -mrelocation-model static -mframe-pointer=none -ffp-contract=on -fno-rounding-math -mconstructor-aliases -target-feature +atomics -target-feature +bulk-memory -target-feature +mutable-globals -target-feature +sign-ext -target-cpu generic -mllvm -treat-scalable-fixed-error-as-warning -debugger-tuning=gdb -v "-fcoverage-compilation-dir=D:\\emscripten" -resource-dir "D:\\emsdk\\3.1.20\\upstream\\lib\\clang\\16.0.0" -D __EMSCRIPTEN_SHARED_MEMORY__=1 -D EMSCRIPTEN -I "D:\\emscripten\\cache\\sysroot\\include\\SDL" -isysroot "D:\\emscripten\\cache\\sysroot" -internal-isystem "D:\\emscripten\\cache\\sysroot/include/wasm32-emscripten/c++/v1" -internal-isystem "D:\\emscripten\\cache\\sysroot/include/c++/v1" -internal-isystem "D:\\emsdk\\3.1.20\\upstream\\lib\\clang\\16.0.0\\include" -internal-isystem "D:\\emscripten\\cache\\sysroot/include/wasm32-emscripten" -internal-isystem "D:\\emscripten\\cache\\sysroot/include" -std=c++17 -fdeprecated-macro "-fdebug-compilation-dir=D:\\emscripten" -ferror-limit 19 -fmessage-length=176 -fvisibility=default -pthread -fgnuc-version=4.2.1 -fcxx-exceptions -fignore-exceptions -fexceptions -fcolor-diagnostics "-iwithsysroot/include\\compat" -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -o "C:\\Users\\kdots\\AppData\\Local\\Temp\\emscripten_temp_rxxb846o\\test_0.o" -x c++ D:/build/test.cpp
clang -cc1 version 16.0.0 based upon LLVM 16.0.0git default target x86_64-pc-windows-msvc
ignoring nonexistent directory "D:\emscripten\cache\sysroot/include/wasm32-emscripten/c++/v1"
ignoring nonexistent directory "D:\emscripten\cache\sysroot/include/wasm32-emscripten"
#include "..." search starts here:
#include <...> search starts here:
D:\emscripten\cache\sysroot\include\SDL
D:\emscripten\cache\sysroot/include\compat
D:\emscripten\cache\sysroot/include/c++/v1
D:\emsdk\3.1.20\upstream\lib\clang\16.0.0\include
D:\emscripten\cache\sysroot/include
End of search list.
"D:/emsdk/3.1.20/upstream/bin\wasm-ld.exe" -o D:/build/test.wasm C:\Users\kdots\AppData\Local\Temp\emscripten_temp_rxxb846o\test_0.o -LD:\emscripten\cache\sysroot\lib\wasm32-emscripten D:\emscripten\cache\sysroot\lib\wasm32-emscripten\crtbegin.o --whole-archive -lwasmfs-mt-debug --no-whole-archive -lGL-mt -lal -lhtml5 -lstubs-debug -lc-mt-debug -ldlmalloc-mt -lcompiler_rt-mt -lc++-mt-noexcept -lc++abi-mt-noexcept -lsockets-mt -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --import-undefined --import-memory --shared-memory --strip-debug --export-if-defined=main --export-if-defined=_emscripten_thread_init --export-if-defined=_emscripten_thread_exit --export-if-defined=_emscripten_thread_crashed --export-if-defined=_emscripten_tls_init --export-if-defined=pthread_self --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=__main_argc_argv --export-if-defined=fflush --export=emscripten_stack_get_end --export=emscripten_stack_get_free --export=emscripten_stack_get_base --export=emscripten_stack_init --export=_wasmfs_read_file --export=stackSave --export=stackRestore --export=stackAlloc --export=__wasm_call_ctors --export=__errno_location --export=emscripten_dispatch_to_thread_ --export=_emscripten_thread_free_data --export=emscripten_main_browser_thread_id --export=emscripten_main_thread_process_queued_calls --export=emscripten_run_in_main_runtime_thread_js --export=emscripten_stack_set_limits --export=emscripten_proxy_finish --export=__get_temp_ret --export=__set_temp_ret --export=__funcs_on_exit --export=malloc --export=free --export=__cxa_is_pointer_type --export-table -z stack-size=5242880 --initial-memory=16777216 --no-entry --max-memory=16777216 --global-base=1024
"D:/emsdk/3.1.20/upstream\bin\wasm-emscripten-finalize" --dyncalls-i64 --pass-arg=legalize-js-interface-exported-helpers D:/build/test.wasm -o D:/build/test.wasm --detect-features
"D:/emsdk/3.1.20/node/14.18.2_64bit/bin/node.exe" D:\emscripten\src\compiler.js C:\Users\kdots\AppData\Local\Temp\tmp5kas3wnu.json
"D:/emsdk/3.1.20/upstream/bin\llvm-objcopy.exe" D:/build/test.wasm D:/build/test.wasm --remove-section=.debug* --remove-section=producers
"D:/emsdk/3.1.20/node/14.18.2_64bit/bin/node.exe" D:\emscripten\tools\preprocessor.js C:\Users\kdots\AppData\Local\Temp\emscripten_temp_rxxb846o\settings.js worker.js --expandMacros
"D:/emsdk/3.1.20/node/14.18.2_64bit/bin/node.exe" D:\emscripten\tools\preprocessor.js C:\Users\kdots\AppData\Local\Temp\emscripten_temp_rxxb846o\settings.js shell.html
```

Expected output:
```
hello
Finished!
```
Actual output:
```
```