Implement independent instances.

Previously, most of the classes were implemented as singletons so only one
instance was possible.

This change adds an instance handle-based API.  In Java, this API is located
in a different package than the old API (edu.wpi.first.networktables).

Backwards compatibility with ITable and the old NetworkTable API is largely
maintained, but a handful of classes have moved to the new package in Java
(ConnectionInfo and PersistentException), and the old JNI has been completed
replaced.

Also:
- Move SetTeam implementation to Dispatcher.
- Consistently pass time through Java and C++ Value API.
- Rename nt_Value.h to NetworkTableValue.h for consistency with Java.
- Improve documentation
- Make C++ and Java APIs more consistent
- Document RPC functions and support RPC in Java.
- Add polling features for entry and connection listeners and use them to
  move callback threads to Java level.
- Remove thread start and stop hooks (as polling is available).
- Make Notifiers, RpcServer, Dispatcher, and Storage mockable.
- Set NOTIFY_NEW on immediate entry notifications.
- Make GetTable("/") and GetTable("") equivalent.
- Generate local notification for flags update when loading persistent file.

And many unit test updates/changes:
- Use InitGoogleMock instead of InitGoogleTest in test main.
- Move test printers to TestPrinter.h/cpp.
- Provide printers for StringRef, EntryNotifier, and Handle.
- StorageTest: Check notifications.
- Add entry notifier unit tests.
- Storage: Add test for incoming entry assignment.
- Update connection listener tests.
- Add entry listener unit tests.

Fixes #11, #140, #189, #190, #192, #193, #221

manualTests/native/server.cpp

@@ -1,24 +1,31 @@
 #include <chrono>
+#include <climits>
 #include <cstdio>
 #include <thread>
 
 #include "ntcore.h"
 
 int main() {
-  nt::SetLogger(
-      [](unsigned int level, const char* file, unsigned int line,
-         const char* msg) {
-        std::fputs(msg, stderr);
+  auto inst = nt::GetDefaultInstance();
+  nt::AddLogger(
+      inst,
+      [](const nt::LogMessage& msg) {
+        std::fputs(msg.message.c_str(), stderr);
         std::fputc('\n', stderr);
       },
-      0);
-  nt::StartServer("persistent.ini", "", 10000);
+      0, UINT_MAX);
+  nt::StartServer(inst, "persistent.ini", "", 10000);
   std::this_thread::sleep_for(std::chrono::seconds(1));
-  nt::SetEntryValue("/foo", nt::Value::MakeDouble(0.5));
-  nt::SetEntryFlags("/foo", NT_PERSISTENT);
-  nt::SetEntryValue("/foo2", nt::Value::MakeDouble(0.5));
-  nt::SetEntryValue("/foo2", nt::Value::MakeDouble(0.7));
-  nt::SetEntryValue("/foo2", nt::Value::MakeDouble(0.6));
-  nt::SetEntryValue("/foo2", nt::Value::MakeDouble(0.5));
+
+  auto foo = nt::GetEntry(inst, "/foo");
+  nt::SetEntryValue(foo, nt::Value::MakeDouble(0.5));
+  nt::SetEntryFlags(foo, NT_PERSISTENT);
+
+  auto foo2 = nt::GetEntry(inst, "/foo2");
+  nt::SetEntryValue(foo2, nt::Value::MakeDouble(0.5));
+  nt::SetEntryValue(foo2, nt::Value::MakeDouble(0.7));
+  nt::SetEntryValue(foo2, nt::Value::MakeDouble(0.6));
+  nt::SetEntryValue(foo2, nt::Value::MakeDouble(0.5));
+
   std::this_thread::sleep_for(std::chrono::seconds(10));
 }