Refactor Transaction API (and break client-java dependency to server)…

… (#4977)

## What is the goal of this PR?

Issue #4513: we need to remove any direct and transitive dependencies from //client-java (`grakn.core.client`) to //server (`grakn.core.server` and `grakn.core.graql`). In order to do that, we need to split Grakn Transaction API (`Session`, `Transaction`, `Keyspace` interfaces`) into a separate module, which does not carry over any external dependency transitively. 

## What are the changes implemented in this PR?

Continuing from PR #4976, this PR fixes #4513, by completing the following changes.
- Replaced all usage of `Transaction` in `//server` and tests with `TransactionOLTP`
- Replaced all usage of `Session` in `//server` and tests with `SessionImpl`
- Replaced the use of `Transaction.Type.<READ|WRITE>` to open transactions with simpler and more robust API: `session.transaction().read()` and `session.transaction().write()`. This avoids client and server transaction users to import and use `Transaction.Type` class which quite frankly complicates the architecture shared between `//client` and `//server`.
- Extracted `Session`, `Transaction` into its own package `//api` (along with `Keyspace` which was moved there in the previous PR).
- Removed `//client-java` dependency onto `//server` - finally.

ConsoleSession.java

@@ -99,7 +99,7 @@ void load(Path filePath) throws IOException {
         consoleReader.println("...");
         consoleReader.flush();
 
-        tx = session.transaction(GraknClient.Transaction.Type.WRITE);
+        tx = session.transaction().write();
 
         try {
             String queries = readFile(filePath);
@@ -118,7 +118,7 @@ void run() throws IOException, InterruptedException {
         consoleReader.setExpandEvents(false); // Disable JLine feature when seeing a '!'
         consoleReader.print(COPYRIGHT);
 
-        tx = session.transaction(GraknClient.Transaction.Type.WRITE);
+        tx = session.transaction().write();
         String queryString;
 
         while ((queryString = consoleReader.readLine()) != null) {
@@ -250,7 +250,7 @@ private void clean() throws IOException {
 
     private void reopenTransaction() {
         if (!tx.isClosed()) tx.close();
-        tx = session.transaction(GraknClient.Transaction.Type.WRITE);
+        tx = session.transaction().write();
     }
 
     @Override