Incorrect behavior if *_GAS_PRICE_UPDATE_INTERVAL is not defined

[akolotov]

If there is no gas price oracle for particular network it seems logical to not define any of these parameters:

*_GAS_PRICE_ORACLE_URL=
*_GAS_PRICE_SPEED_TYPE=
*_GAS_PRICE_UPDATE_INTERVAL=

At the same time it makes sense to configure *_GAS_PRICE_FALLBACK assuming that it will be used if the gas price values will not be available from the contract.

If such configuration is used the following code does not work properly since updateInterval is undefined.

  fetchGasPriceInterval = setInterval(async () => {                                                                                                                                                         
    const gasPrice = await fetchGasPrice({                                                                                                                                                                  
      bridgeContract,                                                                                                                                                                                       
      oracleFn: () => fetchGasPriceFromOracle(oracleUrl, speedType)                                                                                                                                         
    })                                                                                                                                                                                                      
    cachedGasPrice = gasPrice || cachedGasPrice                                                                                                                                                             
    logger.info(`Updated gas price: ${cachedGasPrice}`)                                                                                                                                                     
  }, updateInterval)                                                                                                                                                                                        

Consider to initialize updateInterval in some default value if *_GAS_PRICE_UPDATE_INTERVAL is not configured.

[akolotov]

Addressed in #95