refactor: init epoch to 1 and add a note to make it clear why this is that way

Author: abarmat

contracts/epochs/EpochManager.sol

@@ -29,7 +29,9 @@ contract EpochManager is EpochManagerV1Storage, GraphUpgradeable, IEpochManager
 
         Managed._initialize(_controller);
 
-        lastLengthUpdateEpoch = 0;
+        // NOTE: We make the first epoch to be one instead of zero to avoid any issue
+        // with composing contracts that may use zero as an empty value
+        lastLengthUpdateEpoch = 1;
         lastLengthUpdateBlock = blockNum();
         epochLength = _epochLength;
 
@@ -104,7 +106,7 @@ contract EpochManager is EpochManagerV1Storage, GraphUpgradeable, IEpochManager
      * @return The current epoch based on epoch length
      */
     function currentEpoch() public view override returns (uint256) {
-        return lastLengthUpdateEpoch.add(epochsSinceUpdate()).add(1);
+        return lastLengthUpdateEpoch.add(epochsSinceUpdate());
     }
 
     /**

test/epochs.test.ts

@@ -68,6 +68,11 @@ describe('EpochManager', () => {
     })
 
     describe('calculations', () => {
+      it('first epoch should be 1', async function () {
+        const currentEpoch = await epochManager.currentEpoch()
+        expect(currentEpoch).eq(1)
+      })
+
       it('should return correct block number', async function () {
         const currentBlock = await latestBlock()
         expect(await epochManager.blockNum()).eq(currentBlock)