Fix #115 Concurrency issue when adding documents.

[niemyjski]

Fixes #115

System.InvalidOperationException: Operations that change non-concurrent collections must have exclusive access. A concurrent update was performed on this collection and corrupted its state. The collection's state is no longer correct.
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at System.Collections.Generic.Dictionary`2.set_Item(TKey key, TValue value)
   at Foundatio.Repositories.Elasticsearch.Configuration.DailyIndex.EnsureDateIndexAsync(DateTime utcDate) in /_/src/Foundatio.Repositories.Elasticsearch/Configuration/DailyIndex.cs:line 150
   at Foundatio.Repositories.Elasticsearch.ElasticRepositoryBase`1.IndexDocumentsAsync(IReadOnlyCollection`1 documents, Boolean isCreateOperation, ICommandOptions options) in /_/src/Foundatio.Repositories.Elasticsearch/Repositories/ElasticRepositoryBase.cs:line 1252
   at Foundatio.Repositories.Elasticsearch.ElasticRepositoryBase`1.AddAsync(IEnumerable`1 documents, ICommandOptions options) in /_/src/Foundatio.Repositories.Elasticsearch/Repositories/ElasticRepositoryBase.cs:line 93
   at Foundatio.Repositories.Elasticsearch.ElasticRepositoryBase`1.AddAsync(T document, ICommandOptions options) in /_/src/Foundatio.Repositories.Elasticsearch/Repositories/ElasticRepositoryBase.cs:line 67

[niemyjski]

It kind of feels like we should be locking on the index name here, but this allows us to go wide. I'm wondering if we should also change this to a concurrent dictionary.