Eth2 client: naming improvements

[sept-en]

eth2-client contains some unclear and ambiguous variable names:

• unfinalized_headers could either mean execution headers or beacon headers.
• network could either mean the NEAR network that the contract is deployed to or the Ethereum network that the client watches.
• submitters could be interpreted as likely a list of registered submitters, num_submitted_headers_by_submitter may be a more appropriate name.
• max_submitted_headers refers to the number of unfinalized headers that the relay should submit in one batch but could be interpreted as coupled to max_submitted_blocks_by_account; headers_batch_size may be a more appropriate name.

Additionally, the variable light_client_updates_submission_frequency_in_epochs is specified to represent "N submissions per epoch," which is technically a period (the inverse of the frequency).

[olga24912]

unfinalized_headers -- should be renamed in the Eth2 Light Client contract

[olga24912]

submitters -- it is story about contract as well

[sept-en]

@olga24912 exactly, that issue is about the contract.