Refactor pooled CharBuffer usages within the agent (RabbitMQ body capture)

## Description
Found while trying to add RabbitMQ body capture when working on https://github.com/elastic/apm-agent-java/pull/1328.
While not strictly required to implement RabbitMQ message capture, it would greatly improve implementation and avoid any bug-sensitive code duplication.

- RabbitMQ message is provided as a `byte[]` and we currently only allow for `String` messages (used in Kafka and JMS instrumentations)
- Building a `String` from the `byte[]` would incur extra memory allocation, thus making the `Message` class to use a pooled `CharBuffer` as it's currently done with the `Request` class seems the right option.
- Dealing with `CharBuffer` requires explicit cast to `Buffer` for some operations to provide compatibility with Java 7,8 and 9+ JDKs (see https://github.com/elastic/apm-agent-java/issues/400). For example `CharBuffer#flip` would return `Buffer` in Java7 and 8 (as it's directly inherithed from `Buffer`), and is overloaded [in Java9+ and returns `CharBuffer`](https://docs.oracle.com/javase/9/docs/api/java/nio/CharBuffer.html#flip--) which breaks binary compatibility unless there is an explicit cast to `Buffer`.

## Proposal

Creating a new class named `PooledCharBuffer` that would be used to hold the `body` field of `Message` and `Request` classes.

- a simple `String` value when the buffer is not required (would be used for JMS/Kafka)
- a `CharBuffer` when buffering is required (would be used for RabbitMQ/HTTP request capture)
- write operations that would be delegated to the internal `CharBuffer` without exposing it directly
- read operation `asCharSequence()` that would just return the internal `String` or `CharBuffer` depending on which one is used

Also, this could also be used for SQL statement capture as used in the JDBC instrumentation plugin.

Here is a draft implementation of such class

```
package co.elastic.apm.agent.impl.context;

import co.elastic.apm.agent.objectpool.Allocator;
import co.elastic.apm.agent.objectpool.ObjectPool;
import co.elastic.apm.agent.objectpool.Recyclable;
import co.elastic.apm.agent.objectpool.Resetter;
import co.elastic.apm.agent.objectpool.impl.QueueBasedObjectPool;
import co.elastic.apm.agent.report.serialize.DslJsonSerializer;
import org.jctools.queues.atomic.MpmcAtomicArrayQueue;

import javax.annotation.Nullable;
import java.nio.Buffer;
import java.nio.CharBuffer;

public class PooledBuffer implements Recyclable {

// TODO the pool itself should not be static and should have configurable capacity and allocation sizes
private static final ObjectPool charBufferPool = QueueBasedObjectPool.of(new MpmcAtomicArrayQueue(128), false,
new Allocator() {
@Override
public CharBuffer createInstance() {
return CharBuffer.allocate(DslJsonSerializer.MAX_LONG_STRING_VALUE_LENGTH);
}
},
new Resetter() {
@Override
public void recycle(CharBuffer object) {
((Buffer) object).clear();
}
});

@Nullable
private CharBuffer buffer;

private boolean bufferWriteFinished;

@Nullable
private String stringContent;

public PooledBuffer() {
}

public PooledBuffer write(String content) {
this.stringContent = content;
return this;
}

public PooledBuffer startWriteBuffer() {
checkWriteState(true);
this.bufferWriteFinished = false;
return this;
}

public PooledBuffer appendBuffer(CharSequence s) {
checkWriteState(false);
buffer.append(s);
return this;
}

public PooledBuffer endWriteBuffer() {
checkWriteState(false);
((Buffer) buffer).flip();
bufferWriteFinished = true;
return this;
}

@Nullable
public CharSequence asCharSequence() {
if (stringContent != null) {
return stringContent;
}
return bufferWriteFinished ? buffer : null;
}

public void copyOf(PooledBuffer other) {
// TODO
}

@Override
public void resetState() {
this.stringContent = null;
if (buffer != null) {
charBufferPool.recycle(buffer);
buffer.clear();
}
this.bufferWriteFinished = false;
}

private void checkWriteState(boolean expectFinished) {
if (bufferWriteFinished != expectFinished) {
throw new IllegalStateException("unexpected buffer write state");
}
}
}

```