Get a block by its Block Identifier. If transactions are returned in
the same call to the node as fetching the block, the response should
include these transactions in the Block object. If not, an array of
Transaction Identifiers should be returned so /block/transaction
fetches can be done to get all transaction information.

When requesting a block by the hash component of the BlockIdentifier,
this request MUST be idempotent: repeated invocations for the same
hash-identified block must return the exact same block contents.

No such restriction is imposed when requesting a block by height,
given that a chain reorg event might cause the specific block at
height n to be set to a different one.