Fix global threads count var update

[alonre24]

Update the global variable that indicates the number of RedisAI background threads from the main thread to ensure synchronization upon creating the global ONNX structure. When ONNX model is loaded via AI.MODELSTORE command for the first time, it triggers loading of ONNX backend and allocation of the global array, whose size equals to the number of background threads. Previously, the BG threads themselves where in charge of updating this counter, and there was a race condition that may lead to server crash - https://redislabs.atlassian.net/browse/MOD-3262. This PR is the fix for that issue.

[codecov]

Codecov Report

Merging #923 (12a9ccc) into master (2dd4ef1) will increase coverage by 0.44%.
The diff coverage is 96.20%.

@@            Coverage Diff             @@
##           master     #923      +/-   ##
==========================================
+ Coverage   81.30%   81.75%   +0.44%     
==========================================
  Files          54       54              
  Lines        8201     8220      +19     
==========================================
+ Hits         6668     6720      +52     
+ Misses       1533     1500      -33     

Impacted Files	Coverage Δ
src/execution/DAG/dag_builder.c	100.00% <ø> (ø)
src/execution/DAG/dag_op.c	100.00% <ø> (ø)
src/execution/parsing/script_commands_parser.c	78.57% <ø> (-0.12%)	⬇️
.../serialization/RDB/decoder/previous/v2/decode_v2.c	46.37% <50.00%> (+0.45%)	⬆️
tests/module/LLAPI.c	75.89% <80.00%> (-0.02%)	⬇️
src/execution/background_workers.c	94.63% <95.23%> (+0.01%)	⬆️
src/redisai.c	89.08% <96.77%> (+1.29%)	⬆️
...backends/libtorch_c/torch_extensions/torch_redis.h	90.90% <100.00%> (ø)
src/backends/tensorflow.c	72.43% <100.00%> (+0.06%)	⬆️
src/execution/DAG/dag.c	91.50% <100.00%> (-0.28%)	⬇️
... and 16 more

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