Add MessageId::batchSize() and the MessageIdBuilder

[BewareMyPower]

Master issue: #87

Motivation

To support batch index acknowledgment, we must provide a method to get the batch size of a batched message ID.

Modifications

Instead of adding another overload constructor to MessageId, this PR adds a MessageIdBuilder class to construct the MessageId in a more elegant way.

The original constructor is counterintuitive because the partition index is the 1st argument.

pulsar-client-cpp/include/pulsar/MessageId.h

Line 47 in 74ef1a0

explicit MessageId(int32_t partition, int64_t ledgerId, int64_t entryId, int32_t batchIndex);

Therefore, this PR marks it as deprecated and replace all invocations of it with the MessageIdBuilder usages.

To verify the MessageId::batchSize(), the following tests are modified:

• BatchMessageTest.testBatchSizeInBytes: the batch size is always 2 because of the batchingMaxAllowedSizeInBytes config.
• MessageChunkingTest.testEndToEnd: the batch size field is not set
  (default: 0) because batching is disabled.

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[BewareMyPower]

ping @merlimat @RobertIndie @Demogorgon314

[BewareMyPower]

@merlimat @RobertIndie @Demogorgon314 PTAL. Though I opened a discussion here and reached a concensus that Java's MessageId is an opaque, the MessageId in C++ is still constructable from application side.

This PR mainly changes the construction from

// partition: 1, ledger: 2, entry: 3, batch index: 4
MessageId id(1, 2, 3, 4);

to

auto id = MessageIdBuilder().ledgerId(2).entryId(3).partition(1).batchIndex(4).build();

And support batch_size field.