Fix thread safety issue in Symmetric cipher

[micahlee]

Desired Outcome

The desired outcome of this PR is to allow thread-safe use of the Symmetric cipher class.

Implemented Changes

This PR adds a synchronization mutex to the Symmetric class to manage thread
access to the critical portions of OpenSSL encryption and decryption.

Previously, if this steps were allowed to interleave among threads, it would corrupt
the OpenSSL state, leading to intermittent CipherErrors from OpenSSL.

Connected Issue/Story

CyberArk internal issue ID: CNJR-479

Definition of Done

At least 1 todo must be completed in the sections below for the PR to be
merged.

Changelog

• The CHANGELOG has been updated, or
• This PR does not include user-facing changes and doesn't require a
  CHANGELOG update

Test coverage

• This PR includes new unit and integration tests to go with the code
  changes, or
• The changes in this PR do not require tests

Documentation

• Docs (e.g. READMEs) were updated in this PR
• A follow-up issue to update official docs has been filed here: [insert issue ID]
• This PR does not require updating any documentation

Behavior

• This PR changes product behavior and has been reviewed by a PO, or
• These changes are part of a larger initiative that will be reviewed later, or
• No behavior was changed with this PR

Security

• Security architect has reviewed the changes in this PR,
• These changes are part of a larger initiative with a separate security review, or
• There are no security aspects to these changes

[jvanderhoof]

Looks good. Any idea if there are other parts of Slosilo that call out to OpenSSL that may suffer similar problems?

[micahlee]

Looks good. Any idea if there are other parts of Slosilo that call out to OpenSSL that may suffer similar problems?

@jvanderhoof Thanks! I didn't see any other issues like this in a quick survey through the code. All of the other uses of OpenSSL seem to be single function calls, or use this Symmetric class.