Add InitFlow and move session states to context

[mirromutth]

Motivation:

Session states are now dispersed across MySqlSimpleConnection and ConnectionContext, make these states to be managed in a single object.

Modification:

• Add InitFlow for initialize handshake/login and session states
• Add setSessionVariable and setSessionVariables to QueryFlow
• Consolidate session variable statements into StringUtils
• Move session states to ConnectionConext, e.g. PrepareCache, lockWaitTimeout, etc.
  • All session states should be reset/recreate after reconnect or redirect, see also #89 , #228
  • Remove ConnectionState, because all states should be used by ConnectionContext

Result:

Refactored initialization and management of session states

[jchrys]

I just had a medical check-up; I'll review PRs tomorrow.

[jchrys]

Great change 👍