fix(model-server): writing to a branch using the v1 API failed

Ignite doesn't support nested transactions. Starting a transaction from
inside a transaction throws the following exception:

java.lang.IllegalStateException: Failed to start new transaction (current thread already has a transaction): ...
	at org.apache.ignite.internal.processors.cache.transactions.IgniteTransactionsImpl.txStart0(IgniteTransactionsImpl.java:184)
	at org.apache.ignite.internal.processors.cache.transactions.IgniteTransactionsImpl.txStart(IgniteTransactionsImpl.java:70)
	at org.modelix.model.server.store.IgniteStoreClient.runTransaction(IgniteStoreClient.kt:139)
	at org.modelix.model.server.handlers.RepositoriesManager.mergeChanges(RepositoriesManager.kt:151)
	at org.modelix.model.server.handlers.KeyValueLikeModelServer$putEntries$1.invoke(KeyValueLikeModelServer.kt:323)
	at org.modelix.model.server.handlers.KeyValueLikeModelServer$putEntries$1.invoke(KeyValueLikeModelServer.kt:316)
	at org.modelix.model.server.store.IgniteStoreClient.runTransaction(IgniteStoreClient.kt:140)
	at org.modelix.model.server.handlers.KeyValueLikeModelServer.putEntries(KeyValueLikeModelServer.kt:316)
	at org.modelix.model.server.handlers.KeyValueLikeModelServer$modelServerModule$1$3$7.invokeSuspend(KeyValueLikeModelServer.kt:163)
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
	at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:108)
	at io.netty.util.concurrent.AbstractEventExecutor.runTask(AbstractEventExecutor.java:174)
	at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:167)
	at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:470)
	at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:569)
	at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997)
	at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
	at io.ktor.server.netty.EventLoopGroupProxy$Companion.create$lambda$1$lambda$0(NettyApplicationEngine.kt:296)
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)

skip-lint

Author: slisson

model-server/src/main/kotlin/org/modelix/model/server/store/IgniteStoreClient.kt

@@ -136,10 +136,15 @@ class IgniteStoreClient(jdbcConfFile: File?) : IStoreClient {
 
     override fun <T> runTransaction(body: () -> T): T {
         val transactions = ignite.transactions()
-        transactions.txStart().use { tx ->
-            val result = body()
-            tx.commit()
-            return result
+        if (transactions.tx() == null) {
+            transactions.txStart().use { tx ->
+                val result = body()
+                tx.commit()
+                return result
+            }
+        } else {
+            // already in a transaction
+            return body()
         }
     }