Generalize message flow restrictions

Set of flow restrictions is represented as a "limits" module instance
that can be either created inline via "limits" directive in some modules
(including "remote" target and "smtp" endpoint) or defined globally and
referenced in configuration of modules mentioned above.

This permits a variety of use cases, including shared and separate
counters for various endpoints and also "modules group" style sharing
described in #195.

internal/limits/limiters/limiters.go

@@ -0,0 +1,17 @@
+// Package limiters provides a set of wrappers intended to restrict the amount
+// of resources consumed by the server.
+package limiters
+
+import "context"
+
+// The L interface represents a blocking limiter that has some upper bound of
+// resource use and blocks when it is exceeded until enough resources are
+// freed.
+type L interface {
+	Take() bool
+	TakeContext(context.Context) error
+	Release()
+
+	// Close frees any resources used internally by Limiter for book-keeping.
+	Close()
+}