revert some unnecessary synchronization

revert parts of  21da5178 "Issue #255: ConcurrentModificationException
with multiple threads in embedded mode and temporary LOBs"

There is no need to lock all sessions this aggressively, it's just the
system session we have to be careful with.

h2/src/main/org/h2/engine/Engine.java

@@ -76,7 +76,6 @@ public class Engine implements SessionFactory {
                 DATABASES.put(name, database);
             }
         }
-        synchronized (database) {
         if (opened) {
             // start the thread when already synchronizing on the database
             // otherwise a deadlock can occur when the writer thread
@@ -122,7 +121,6 @@ public class Engine implements SessionFactory {
         }
         return session;
     }
-    }
 
     /**
      * Open a database connection with the given connection information.

h2/src/main/org/h2/engine/Session.java

@@ -650,7 +650,8 @@ public class Session extends SessionWithState {
 
     private void removeTemporaryLobs(boolean onTimeout) {
         if (SysProperties.CHECK2) {
-            if (!Thread.holdsLock(this) && !Thread.holdsLock(getDatabase())) {
+            if (this == getDatabase().getLobSession()
+                    && !Thread.holdsLock(this) && !Thread.holdsLock(getDatabase())) {
                 throw DbException.throwInternalError();
             }
         }

h2/src/main/org/h2/engine/SessionRemote.java

@@ -581,9 +581,7 @@ public class SessionRemote extends SessionWithState implements DataHandler {
         }
         traceSystem.close();
         if (embedded != null) {
-            synchronized (embedded) {
             embedded.close();
-            }
             embedded = null;
         }
         if (closeError != null) {

h2/src/main/org/h2/server/TcpServerThread.java

@@ -181,9 +181,7 @@ public class TcpServerThread implements Runnable {
                 server.traceError(e);
             }
             try {
-                synchronized (session) {
                 session.close();
-                }
                 server.removeConnection(threadId);
             } catch (RuntimeException e) {
                 if (closeError == null) {