Eliminate temporary buffer allocations during bson serialization

[Edarke]

Fixes: JAVA-5786

• Override Outputbuffer.toByteArray() to avoid extraneous array copy
• Modify BsonBinaryWriter to call OutputBuffers.writeObjectId so implementations can opt to use putToByteBuffer rather than allocating a temporary byte[]
• Wrap array in ByteBuffer so we can write numeric values 8 bytes at a time

Motivation

Atlas Search uses the Java driver to communicate with the Mongodb Server. We serialize up to ~300k documents per batch, primarily consisting of ObjectID and numbers. For these large batches, serialization has measurable overhead. This PR proposes some simple optimizations for BasicOutputBuffer without altering the API.

JMH Benchmarks

Benchmark	Main (ops/ms)	GC (B/op)	This PR (ops/ms)	GC (B/op)
toByteArray_large	0.907	26,777,857	2.269	13,388,928
toByteArray_small	110,515	160	184,816	80
encodeId	23,774	134	32,696	104
write_id	28,255	168	36,467	80
write_numbers	16,327	80	18,125	80

[jyemin]

@Edarke please open a Jira issue in the JAVA project for this PR, as per our contribution guide.

Thanks!

[vbabanin]

Looks good! I just have a few minor comments.

[vbabanin]

Now that the writeInt32(final int value) method has been updated to use ByteBuffer's bulk operation putInt(...), this method is no longer covered by tests. Previously, it was indirectly tested through writeInt32. Could you add a dedicated test case for this?

[Edarke]

Good catch, updated in 9a187bd