Multiple enclave keys support

[bluele]

Core

• introduce a new Enclave Key Manager that uses SQLite
  • It was chosen because it is desirable that there can be multiple writer processes and be able to use flexible query
• use sgx_tseal as sealing library
  • Because of the change from file to DB management

Relayer

• relayer now checks the expiry date of the keys it uses
• also fetches a list of available keys from the LCP service and selects the appropriate one
• add key_expiration field to config

CLI

• removed the following commands
  • enclave init-key
  • enclave show-avr
• instead, added the following
  • enclave generate-key: generate a new enclave key
  • enclave list-keys: show available keys
  • enclave prune-keys: prune expired keys