Improved Scheduler API (#696)

* Improved Scheduler API

- Added `Scheduler#builder(plugin)`
This method allows a more simplified builder while maintaining the main requirement of the executor plugin
- Added `Scheduler#taskByPlugin(plugin)`
Allows to obtain the tasks that a plugin has sent to execute and that are currently active
- Added `TaskBuilder#task(Consumer<SchuledTask>)`
Allows to specify a task with access to the task itself with the ability to cancel itself

* Applied requested changes

- Removed tasks builder method
- Added `Scheduler#buildTask(plugin, Consumer<ScheduledTask>)`

* Removed some unused imports

* Applied suggested change

* Fix possible test bug

* Applied more suggested changes

* Fixed tests inside tasks

native/src/main/java/com/velocitypowered/natives/compression/JavaVelocityCompressor.java

@@ -20,7 +20,6 @@ package com.velocitypowered.natives.compression;
 import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.base.Preconditions.checkState;
 import static com.velocitypowered.natives.compression.CompressorUtils.ZLIB_BUFFER_SIZE;
-import static com.velocitypowered.natives.compression.CompressorUtils.ensureMaxSize;
 
 import com.velocitypowered.natives.util.BufferPreference;
 import io.netty.buffer.ByteBuf;

native/src/test/java/com/velocitypowered/natives/compression/VelocityCompressorTest.java

@@ -34,9 +34,7 @@ import java.util.zip.DataFormatException;
 import java.util.zip.Deflater;
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
-import org.junit.jupiter.api.condition.EnabledOnJre;
 import org.junit.jupiter.api.condition.EnabledOnOs;
-import org.junit.jupiter.api.condition.JRE;
 
 class VelocityCompressorTest {