Allow the repo locker to be passed by configuration

[jacobheun]

Currently the only available lockers are fs and memory. It's currently not possible to set a locker via configuration. Providing support for this would allow new datastores to provide their own locking mechanism.

Perhaps updating the Repo initialization to include a locker property would be a good approach:

const repo = new Repo({
  locker: require('./my-locker')
})

Then when setting this._locker internally, it could check for either this.options.locker, or the fs|memory keys in this.options.lock

connects to ipfs/js-datastore-s3#1

[daviddias]

Thanks for bringing this up. I agree there is a good case to make the lock file optional and even avoiding using fs-ext to lock on fs (which causes problems to electron). From my side:

• 👍 on making the lock optional and configurable.
• 👍 on abandoning fs-ext.