Use BigDecimal to improve accracy of TPS results - long live 20 TPS!

It's always been commonly said to 'ignore' that TPS was '19.X', that
it was fine.

I suspect that the inaccuracy of floating point math resulted in us
losing precision over time, making it difficult to actually get back to 20,
as you know the fun 0.1 + 0.1 ... 9 more times != 1 problem.

BigDecimal supports working with doubles with higher precision.

This change makes it so our RollingAverage class maintains all of the data
using BigDecimal and using BigDecimal arithematic operations.

This ensures we have extremely high precision, enabling us to
actually be able print '20 TPS' when TPS is perfect.

Spigot-Server-Patches/Optimize-explosions.patch

@@ -124,7 +124,7 @@ index 2620c4c2bd..a31e07b903 100644
 +    // Paper end
  }
 diff --git a/src/main/java/net/minecraft/server/MinecraftServer.java b/src/main/java/net/minecraft/server/MinecraftServer.java
-index 30541dfa5a..bbd476bb0e 100644
+index 5086fe4027..abed6bb977 100644
 --- a/src/main/java/net/minecraft/server/MinecraftServer.java
 +++ b/src/main/java/net/minecraft/server/MinecraftServer.java
 @@ -0,0 +0,0 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
@@ -136,7 +136,7 @@ index 30541dfa5a..bbd476bb0e 100644
          }
  
 diff --git a/src/main/java/net/minecraft/server/World.java b/src/main/java/net/minecraft/server/World.java
-index 0fe8a97f24..c8b9c10f12 100644
+index e06da6bef9..c823cef341 100644
 --- a/src/main/java/net/minecraft/server/World.java
 +++ b/src/main/java/net/minecraft/server/World.java
 @@ -0,0 +0,0 @@ import org.apache.logging.log4j.Logger;