Rework gas explosion code

[Erlend Åmdal]

Gas explosions are extremely heavy to compute since each block of gas is having its own tiny explosion. In addition, this causes gas explosions to barely damage the player while still being capable of throwing them 200 blocks into the air.

To counter this, gas explosions should make fewer, larger explosions by combining the explosive power of surrounding explosive gases.

[Trent VanSlyke]

Recommended approach: these fewer, larger explosions should have a 0.5-1.5 second delay between explosions. This eases the load of explosions, and creates a nice "boom, boom, boom, boom" sound effect. Also allows the player to see the explosion incoming and react.