kms: extend the `Client.Restart` API to support cluster-wide restarts (…

…minio#40)

This commit changes the behavior of `Client.Restart` such that it
uses the cluster-wide restart mechanism to restart all nodes within
a cluster.

The adv. of the new cluster-wide restart is that a client doesn't have
to know all cluster nodes to restart all nodes within a cluster. Instead,
it can delegate this task to one of the cluster nodes.

However, a client can still restart all cluster nodes itself by providing
a list of hosts.

Signed-off-by: Andreas Auernhammer <github@aead.dev>

kes: use global `math/rand` instead of `math/rand.New` (minio#26)

This commit reverts to the global `math/rand` RNG.
The global RNG is safe for current use - due to its source.
The default `rand.NewSource` is not safe for concurrent usage.

This can cause crashes like the following:
```
panic: runtime error: index out of range [-1]

goroutine 3902089 [running]:
math/rand.(*rngSource).Uint64(...)
	math/rand/rng.go:249
math/rand.(*rngSource).Int63(0x0?)
	math/rand/rng.go:234 +0x85
math/rand.(*Rand).Int63(...)
	math/rand/rand.go:96
math/rand.(*Rand).Int31(...)
	math/rand/rand.go:110
math/rand.(*Rand).Int31n(0x454c860?, 0x1?)
	math/rand/rand.go:142 +0x6a
math/rand.(*Rand).Intn(0xc00f099020?, 0x1d?)
	math/rand/rand.go:183 +0x25
github.com/minio/kms-go/kes.(*loadBalancer).Send(0xc000afeba0, {0x68bcb58, 0x91fb380}, 0xc0192ce720, {0x4805358, 0x4},
```

Signed-off-by: Andreas Auernhammer <github@aead.dev>

kms: add support for stack trace levels (minio#20)

This commit adds support for requesting stack traces
only for specific log records.
Now a client can request stack traces only for records
with a certain log level. This allows a client to filter
out non-interesting stack traces (e.g. for debug log records).

While clients can always filter themselves, not sending
them at the server-side saves bandwidth and CPU time used
when marshaling.

Signed-off-by: Andreas Auernhammer <github@aead.dev>

Update README.md

kes: add `Client.HMAC`

This commit adds the `Client.HMAC` method that
computes the HMAC of a message using a key at
the KES server.

kes: add `Client.HMAC`

This commit adds the `Client.HMAC` method that
computes the HMAC of a message using a key at
the KES server.

kms: add more APIs and request/response types.

This commit adds support for more KMS APIs, like
creating and deleting enclaves and secret keys.

It also adds the generic `ListRequest` and `ListResponse`
types.

Signed-off-by: Andreas Auernhammer <github@aead.dev>

add encrypt, decrypt and generate key APIs

This commit adds the encrypt, decrypt and generate data encryption
key APIs.

Signed-off-by: Andreas Auernhammer <github@aead.dev>

fix argument order in `policy.Verify`

This commit fixes a bug in the `policy.Verify` function.
The order of the arguments was incorrect causing `Verify`
to behave incorrectly.

This commit also adds some test cases to test this behavior.

Signed-off-by: Andreas Auernhammer <github@aead.dev>

prepare SDK for the distributed KES implementation

This commit changes the SDK w.r.t. to the upcoming
distributed KES server implementation.

In particular, the listing code has been changed
to a paginated implementation. However, backwards
compatibility with existing KES servers is maintained.

Signed-off-by: Andreas Auernhammer <hi@aead.dev>