[#326] ir: Implement settlement processor

Define a processor of events related to monetary transactions. Define
audit-related event. Provide an interface for processing the audit payout
event.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/innerring/processors/settlement/processor.go

@@ -0,0 +1,58 @@
+package settlement
+
+import (
+	"fmt"
+
+	nodeutil "github.com/nspcc-dev/neofs-node/pkg/util"
+	"github.com/nspcc-dev/neofs-node/pkg/util/logger"
+	"github.com/panjf2000/ants/v2"
+	"github.com/pkg/errors"
+	"go.uber.org/zap"
+)
+
+// Processor is an event handler for payments in the system.
+type Processor struct {
+	log *logger.Logger
+
+	pool nodeutil.WorkerPool
+
+	auditProc AuditProcessor
+}
+
+// Prm groups the required parameters of Processor's constructor.
+type Prm struct {
+	AuditProcessor AuditProcessor
+}
+
+func panicOnPrmValue(n string, v interface{}) {
+	panic(fmt.Sprintf("invalid parameter %s (%T):%v", n, v, v))
+}
+
+// New creates and returns a new Processor instance.
+func New(prm Prm, opts ...Option) *Processor {
+	switch {
+	case prm.AuditProcessor == nil:
+		panicOnPrmValue("AuditProcessor", prm.AuditProcessor)
+	}
+
+	o := defaultOptions()
+
+	for i := range opts {
+		opts[i](o)
+	}
+
+	pool, err := ants.NewPool(o.poolSize, ants.WithNonblocking(true))
+	if err != nil {
+		panic(errors.Wrap(err, "could not create worker pool"))
+	}
+
+	o.log.Debug("worker pool for settlement processor successfully initialized",
+		zap.Int("capacity", o.poolSize),
+	)
+
+	return &Processor{
+		log:       o.log,
+		pool:      pool,
+		auditProc: prm.AuditProcessor,
+	}
+}