[4.0 -> main] Restore read-mode=speculative

[heifner]

Restore deprecated read-mode=speculative that was removed in #414. read-mode=head remains the default.

Although, read-mode=speculative can cause confusion to users, this PR restores the option as an alternative non-default read-mode. This does not restore persisted trxs. Persisted trxs were API trxs re-applied for each block. This new restored speculative mode therefore will not have the state of API trxs auto applied at the start of each speculative block. It is up to the user to re-apply any transactions at each speculative block in order to observe their side-effects.

Also the restored read-mode=speculative does not, unlike the old removed read-mode=speculative, apply schedule changes, protocol activations, or onblock trxs for a speculative mode into the speculative block which will be aborted when a new block is received from the network. This is in alignment with read-mode=head.

  --read-mode arg (=head)               Database read mode ("head",
                                        "irreversible", "speculative").
                                        In "head" mode: database contains state
                                        changes up to the head block;
                                        transactions received by the node are
                                        relayed if valid.
                                        In "irreversible" mode: database
                                        contains state changes up to the last
                                        irreversible block; transactions
                                        received via the P2P network are not
                                        relayed and transactions cannot be
                                        pushed via the chain API.
                                        In "speculative" mode: (DEPRECATED:
                                        head mode recommended) database
                                        contains state changes by transactions
                                        in the blockchain up to the head block
                                        as well as some transactions not yet
                                        included in the blockchain;
                                        transactions received by the node are
                                        relayed if valid.

Merges #986 into main

Resolves #980