TT-555 initializing session with mutex

[tbuchaillot]

Description

Since we're using pointer to mutex in SessionState, when it's initialized we have to initialize the mutex too.

Related Issue

Issue #3307

Motivation and Context

Just to avoid gw from panicking when the session is loaded from redis.

How This Has Been Tested

1. Start redis docker
2. Start tyk gw, created an api definition and policy
3. Generate a key to access that api
4. Use the key and observe it works fine
5. Stop and start the tyk-gateway (leave redis running) or use /reload
6. Use the key to call the api again

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring or add test (improvements in base code or adds test coverage to functionality)

Checklist

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• My change requires a change to the documentation.
  • If you've changed APIs, describe what needs to be updated in the documentation.
  • If new config option added, ensure that it can be set via ENV variable
• I have updated the documentation accordingly.
• Modules and vendor dependencies have been updated; run go mod tidy && go mod vendor
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• I have added tests to cover my changes.
• All new and existing tests passed.
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