fix for #3090 and #3091

Cause of these issues is that there can be a **synchronization problem** for initializing **attribute type** and using **attribute type** for converting searched value.

**Note:** I could only reproduced this issue with stabilizer (not unit test) with settings mentioned by @Danny-Hazelcast and after this fix, I couldn't see these error never again with diffirent instance, client and worker counts for 30 test executions.

~~~~
Cluster profile: Medium
Stabilizer test: com.hazelcast.stabilizer.tests.map.MapIndexTest,
Hazelcast version: 3.3-RC3
~~~~

Without synchronization around **attribute type**, **Thread-1** can see **attribute type** as **null** since there is no added index and doesn't convert searched value so uses it as raw (String). Then this thread may be **deactivated**, another thread (**Thread-2**) may take a change to run for saving an index and updates **attribute type**. Then **Thread-1** may be **activated** again and iterates over records on index store. In this case, **Thread-1** didn't convert searched value to expected value type and iteration over indexed elements can cause **class cast exception**. To handle this case, synchronization is implemented with **locks** around **attribute type**.
      
In addition, all methods converting searched values and saving index can be defined as **synchronized** and this technique solves the issue, but I think this is **not an efficient way** to making synchronized for all cases since synchronization is required only when attribute type is null. Because in that case, there is no metadata to convert searched value.