Fix lock flag restoration after session reinitialization

[Brokkonaut]

Summary

Fix player-specific weather and time state restoration when sessions are
reinitialized.

Players could remain stuck with the weather configured by a region's
weather-lock flag after leaving the region. This was reproducible when:

• a new session was initialized while the player was inside a weather-locked
  region (for example login after the server was restarted) or
• /wg flushstates was executed while the player was inside such a region.

Moving out of that region would not reset the weather/time to normal afterwards

Cause

New sessions were already initialized by SessionManager#get, but the
ProcessPlayerEvent handler initialized them a second time. During the second
initialization, the weather applied by WorldGuard was recorded as the player's
original weather.

Additionally, Session#resetState initialized handlers again without first
uninitializing them. Stateful handlers could therefore replace their original
player state with the state previously applied by the region flag.

Changes

• Initialize newly created sessions only once.
• Reinitialize cached sessions explicitly on player processing.
• Restore handler-managed player state before reinitializing a session.