Fix hashtable locking

[danielealbano]

This PR fixes a sneaky issue in the hashtable locking, the construct used to get the worker index only once per thread was actually being called only the very first time by only one thread causing all the other threads to have a worker_index of 0.

As consequence the locking was potentially generating overlappy transaction ids leading to a potential clash.

This behaviour was put in evidence by redis-benchmark which uses only 1 single key for all the requests unless the --threads option is passed.

The PR also changes 2 other aspects:

• drops the pthread_once, as it's necessary to check per thread a simple boolean can do the job and simplify the checks and the logic, and also providing a minor but nice speed improvement.
• reduce the amount of spinlock traced by defaylt to reduce the amount of memory allocated, infact the vast majority of cases need just 1 spinlock tracked therefore the size of the list initialized by default has been set to the size of the smallest object that can be initialized by FFMA, 16 bytes.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.03 :warning:

Comparison is base (3eb32e4) 76.83% compared to head (97ace96) 76.81%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #415 +/- ## ========================================== - Coverage 76.83% 76.81% -0.03% ========================================== Files 198 198 Lines 13584 13586 +2 ========================================== - Hits 10437 10435 -2 - Misses 3147 3151 +4 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `76.81% <100.00%> (-0.03%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Daniele+Salvatore+Albano#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://app.codecov.io/gh/danielealbano/cachegrand/pull/415?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Daniele+Salvatore+Albano) | Coverage Δ | | |---|---|---| | [src/transaction.h](https://app.codecov.io/gh/danielealbano/cachegrand/pull/415?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Daniele+Salvatore+Albano#diff-c3JjL3RyYW5zYWN0aW9uLmg=) | `85.71% <ø> (ø)` | | | [src/transaction.c](https://app.codecov.io/gh/danielealbano/cachegrand/pull/415?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Daniele+Salvatore+Albano#diff-c3JjL3RyYW5zYWN0aW9uLmM=) | `94.44% <100.00%> (+0.33%)` | :arrow_up: | ... and [3 files with indirect coverage changes](https://app.codecov.io/gh/danielealbano/cachegrand/pull/415/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Daniele+Salvatore+Albano)

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