Code improvements PoA module #1

[mitsuaki-u]

Expected behavior

1. Misspelled, unused constant . In poa/constants.ts , both
   LENGTH_PROOF_OF_POSSESSION and LENGTH_PROOF_OF_POSESSION are defined,
   both equaling 96. LENGTH_PROOF_OF_POSESSION is never used.

2. Redundant, unused type . In poa/stores/snapshot.ts , the Validator type is
   defined but never used. It is identical to the ActiveValidator type, imported from
   poa/types.ts . ActiveValidator is used later in poa/stores/snapshot.ts .

3. Hardcoded SUBSTORE_PREFIX constants . The LIP specifying the PoA module
   specifies four SUBSTORE_PREFIX constants. These constants are hardcoded in the
   implementation (in poa/module.ts , in the constructor of PoAModule ), rather than
   being defined as constants in poa/constants.ts .

4. Constants are defined but hardcoded versions are used :
   a. In poa/module.ts , line 88, 'poa' is used instead of MODULE_NAME_POA .
   b. In poa/module.ts , line 216, /^[a-z0-9!@$&_.]+$/g is used instead of
   POA_VALIDATOR_NAME_REGEX . While the hardcoded value is slightly
   different from the constant (the hardcoded value includes a g for global at
   the end, while the constant does not), the effect will be the same.
   c. In poa/endpoint.ts , on lines 73 and 74, 20 is used instead of
   NUM_BYTES_ADDRESS .

5. Inconsistent use of Buffer.from . Some code locations use Buffer.from(str,
   'utf-8') while other locations use Buffer.from(str) , leaving out the 'utf-8'
   argument. The effect is the same, since 'utf-8' is the default value of that
   argument

Actual behavior

See above

Steps to reproduce

Check PoA Module

Which version(s) does this affect? (Environment, OS, etc...)

v6.0.0