Optimize parse_block_identifier

[reedsa]

What was wrong?

• Case elif isinstance(block_identifier, bytes) or is_hex_encoded_block_hash(block_identifier) was making multiple calls when it was not necessary
• Return value was using w3.eth.get_block(block_identifier)["number"]

Closes #2816

How was it fixed?

• Separate each case to short circuit when possible
• Returns block_identifier when is_hex_encoded_block_hash is true or returns add_0x_prefix(block_identifier.hex()) when the block_identifier is bytes.

Todo:

• Add entry to the release notes

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