[17449] Introduces a locking mechanism over variables (#18207)

It includes the work over the state store, the PRC server, the HTTP server, the go API package and the CLI's  command. To read more on the actuall functionality, refer to the RFCs [NMD-178] Locking with Nomad Variables and [NMD-179] Leader election using locking mechanism for the Autoscaler.

.changelog/18520.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+vars: Added a locking feature for Nomad Variables
+```

api/api.go

@@ -12,6 +12,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math"
 	"net"
 	"net/http"
 	"net/url"
@@ -200,6 +201,10 @@ type Config struct {
 	TLSConfig *TLSConfig
 
 	Headers http.Header
+
+	// retryOptions holds the configuration necessary to perform retries
+	// on put calls.
+	retryOptions *retryOptions
 }
 
 // ClientConfig copies the configuration with a new client address, region, and
@@ -578,6 +583,40 @@ func (c *Client) SetSecretID(secretID string) {
 	c.config.SecretID = secretID
 }
 
+func (c *Client) configureRetries(ro *retryOptions) {
+
+	c.config.retryOptions = &retryOptions{
+		maxRetries:      defaultNumberOfRetries,
+		maxBackoffDelay: defaultMaxBackoffDelay,
+		delayBase:       defaultDelayTimeBase,
+	}
+
+	if ro.delayBase != 0 {
+		c.config.retryOptions.delayBase = ro.delayBase
+	}
+
+	if ro.maxRetries != defaultNumberOfRetries {
+		c.config.retryOptions.maxRetries = ro.maxRetries
+	}
+
+	if ro.maxBackoffDelay != 0 {
+		c.config.retryOptions.maxBackoffDelay = ro.maxBackoffDelay
+	}
+
+	if ro.maxToLastCall != 0 {
+		c.config.retryOptions.maxToLastCall = ro.maxToLastCall
+	}
+
+	if ro.fixedDelay != 0 {
+		c.config.retryOptions.fixedDelay = ro.fixedDelay
+	}
+
+	// Ensure that a big attempt number or a big delayBase number will not cause
+	// a negative delay by overflowing the delay increase.
+	c.config.retryOptions.maxValidAttempt = int64(math.Log2(float64(math.MaxInt64 /
+		c.config.retryOptions.delayBase.Nanoseconds())))
+}
+
 // request is used to help build up a request
 type request struct {
 	config *Config