Fix a use-after-free condition of single-instance TAs

[sangho2]

This PR fixes a use-after-free condition of single-instance TAs where a CPU core attempts to open a session with a single-instance TA with a cached instance in memory while another is tearing down it (due to session close, TA crash/panic, ...). This PR gets rid of such condition by re-checking whether a cached instance is valid or not. Also, this PR removes a fast pat h for single-instance TA opening which is racy with less meaningful performance benefit.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- failure constructible_struct_adds_field: externally-constructible struct adds field ---

Description:
A pub struct constructible with a struct literal has a new pub field. Existing struct literals must be updated to include the new field.
        ref: https://doc.rust-lang.org/reference/expressions/struct-expr.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.47.0/src/lints/constructible_struct_adds_field.ron

Failed in:
  field TaInstance.dead in /home/runner/work/litebox/litebox/litebox_shim_optee/src/session.rs:125
  field TaInstance.dead in /home/runner/work/litebox/litebox/litebox_shim_optee/src/session.rs:125

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.47.0/src/lints/inherent_method_missing.ron

Failed in:
  SessionManager::get_single_instance, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/e218c7cc0b39b6a0402839f2d17ffb4c681921c3/litebox_shim_optee/src/session.rs:385
  SessionManager::get_single_instance, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/e218c7cc0b39b6a0402839f2d17ffb4c681921c3/litebox_shim_optee/src/session.rs:385