Bump versions for zebrad 1.0.0-alpha.1

[dconnolly]

Tag message when this is merged:

Zebra's second alpha brings multiple reliability and stability improvements for long-running syncs.
We've resolved known panics during syncing, and reduced the number of sync hangs.

Some notable changes include:

Added:
- Add peer set tracing (#1468)
- Add Sentry support behind a feature flag (#1461)
- Log configured network in every log message (#1568)

Changed:
- Export new precompute api in zebra-script (#1493)
- Rewrite peer block request hander to match the zcashd implementation (#1518)

Fixed:
- Avoid panics when there are multiple failures on the same connection (#1600)
- Add sync and inbound timeouts to prevent hangs (#1586) 
- Fix Zebra versions so all crates are on the 1.0.0-alpha series (#1488)
- Make `cargo run` use `zebrad` rather than failing (#1569)
- Panic if the lookahead limit is misconfigured (#1589)
- Recommend using --locked with `cargo install` (#1490)
- Simplify C++ compiler dependency in the README (#1498)
- Stop failing acceptance tests if their directories already exist (#1588)
- Stop panicking when ClientRequests return an error (#1531)
- Upgrade to tokio 0.3.6 to avoid a time wheel panic (#1583, #1511)

Currently, Zebra does not validate all the Zcash consensus rules.

[teor2345]

Looks good!