Represent slots in secs instead of millisecs (#2163)

## Issue Addressed

NA

## Proposed Changes

Copied from #2083, changes the config milliseconds_per_slot to seconds_per_slot to avoid errors when slot duration is not a multiple of a second. To avoid deserializing old serialized data (with milliseconds instead of seconds) the Serialize and Deserialize derive got removed from the Spec struct (isn't currently used anyway).

This PR replaces #2083 for the purpose of fixing a merge conflict without requiring the input of @blacktemplar.

## Additional Info

NA


Co-authored-by: blacktemplar <blacktemplar@a1.net>

lighthouse/environment/tests/minimal_spec/eth2-spec.toml

@@ -1,42 +0,0 @@
-spec_constants = "minimal" # for testing
-
-[spec]
-genesis_slot = 0
-base_rewards_per_epoch = 4
-deposit_contract_tree_depth = 32
-max_committees_per_slot = 64
-target_committee_size = 128
-min_per_epoch_churn_limit = 4
-churn_limit_quotient = 65536
-shuffle_round_count = 90
-min_genesis_active_validator_count = 16384
-min_genesis_time = 1578009600
-min_deposit_amount = 1000000000
-max_effective_balance = 32000000000
-ejection_balance = 16000000000
-effective_balance_increment = 1000000000
-genesis_fork_version = "0x00000000"
-bls_withdrawal_prefix_byte = "0x00"
-genesis_delay = 300
-milliseconds_per_slot = 12000
-min_attestation_inclusion_delay = 1
-min_seed_lookahead = 1
-max_seed_lookahead = 4
-min_epochs_to_inactivity_penalty = 4
-min_validator_withdrawability_delay = 256
-shard_committee_period = 2048
-base_reward_factor = 64
-whistleblower_reward_quotient = 512
-proposer_reward_quotient = 8
-inactivity_penalty_quotient = 33554432
-min_slashing_penalty_quotient = 32
-domain_beacon_proposer = 0
-domain_beacon_attester = 1
-domain_randao = 2
-domain_deposit = 3
-domain_voluntary_exit = 4
-safe_slots_to_update_justified = 8
-eth1_follow_distance = 1024
-seconds_per_eth1_block = 14
-boot_nodes = []
-network_id = 1