api: ranges in txn comparisons

[heyitsanthony]

Currently the Txn API only supports comparisons on single keys. However, comparisons on ranges that check whether all keys match the comparison target are useful for implementing higher-level functionality over etcd.

Disconnected linearized reads would use range comparisons to check for leased keys on a range before deleting a range:

cli.Txn(ctx)
.If(Compare(CreateRevision(leasingPfx + "a", WithCmpRange(leasingPfx + "b")), "=", 0)
.Then(OpDelete("a", WithRange("b"))
.Commit()

STM could be extended to support key intervals with range comparisons to check for read conflicts:

cli.Txn(ctx)
.If(Compare(ModRevision("a", WithCmpRange("b")), "<", maxRangeModRev+1)
.Then(/* put write buffer */)
.Else("failure")
.Commit()

The protocol change would add a range_end field to the Compare protobuf message:

message Compare {
  ...
  oneof target_union {
    // version is the version of the given key
    int64 version = 4;
    // create_revision is the creation revision of the given key
    int64 create_revision = 5;
    // mod_revision is the last modified revision of the given key.
    int64 mod_revision = 6;
    // value is the value of the given key, in bytes.
    bytes value = 7;
  }
  bytes range_end = 8;
}