reader_concurrency_semaphore: add state to permits

Instead of a simple boolean, designating whether the permit was already
admitted or not, add a proper state field with a value for all the
different states the permit can be in. Currently there are three such
states:
* registered - the permit was created and started accounting resource
  consumption.
* waiting - the permit was queued to wait for admission.
* admitted - the permit was successfully admitted.

The state will be used for debugging purposes, both during coredump
debugging as well as for dumping diagnostics data about permits.

reader_concurrency_semaphore.cc

@@ -74,7 +74,7 @@ class reader_permit::impl {
     sstring _op_name;
     std::string_view _op_name_view;
     reader_resources _resources;
-    bool _admitted = false;
+    reader_permit::state _state = reader_permit::state::registered;
 
 public:
     struct value_tag {};
@@ -113,21 +113,29 @@ public:
         return _op_name_view;
     }
 
+    reader_permit::state get_state() const {
+        return _state;
+    }
+
+    void on_waiting() {
+        _state = reader_permit::state::waiting;
+    }
+
     void on_admission() {
-        _admitted = true;
+        _state = reader_permit::state::admitted;
         _semaphore.consume(_resources);
     }
 
     void consume(reader_resources res) {
         _resources += res;
-        if (_admitted) {
+        if (_state == reader_permit::state::admitted) {
             _semaphore.consume(res);
         }
     }
 
     void signal(reader_resources res) {
         _resources -= res;
-        if (_admitted) {
+        if (_state == reader_permit::state::admitted) {
             _semaphore.signal(res);
         }
     }
@@ -147,6 +155,10 @@ reader_permit::reader_permit(reader_concurrency_semaphore& semaphore, const sche
 {
 }
 
+void reader_permit::on_waiting() {
+    _impl->on_waiting();
+}
+
 void reader_permit::on_admission() {
     _impl->on_admission();
 }
@@ -287,6 +299,7 @@ future<reader_permit::resource_units> reader_concurrency_semaphore::do_wait_admi
     }
     promise<reader_permit::resource_units> pr;
     auto fut = pr.get_future();
+    permit.on_waiting();
     _wait_list.push_back(entry(std::move(pr), std::move(permit), r), timeout);
     return fut;
 }

reader_permit.hh

@@ -90,6 +90,12 @@ class reader_permit {
 public:
     class resource_units;
 
+    enum class state {
+        registered, // read is registered, but didn't attempt admission yet
+        waiting, // waiting for admission
+        admitted,
+    };
+
 private:
     class impl;
     shared_ptr<impl> _impl;
@@ -98,6 +104,7 @@ private:
     explicit reader_permit(reader_concurrency_semaphore& semaphore, const schema* const schema, std::string_view op_name);
     explicit reader_permit(reader_concurrency_semaphore& semaphore, const schema* const schema, sstring&& op_name);
 
+    void on_waiting();
     void on_admission();
 
 public: