OAK-11277 Tree store: fix memory usage and support concurrent indexing

[thomasmueller]

The PR fixes a bunch of issues:

• IndexMeta: when re-indexing multiple indexes, each writer that tries to open a writer first reads all metadata files (including those of concurrently added files). The thread is only interested in the current index, and all other indexes are then ignored - and on this index we synchronized. However, the problem is: another thread might concurrently create another index, whose metadata file could be empty when this thread reads it. So protection against that is needed.
• The OakDirectory uses a ConcurrentHashSet, however it doesn't properly synchronize on the node builder.
• The MultiplexingIndexWriter doesn't support concurrent access (unlike the default index writer). This was not detected so far because multi-threaded indexing was usually only used for one single index. I tries reindexing all indexes with many threads, which uncovered this issue. (First, it is using a regular HashMap instead of a concurrent one... but more importantly, it could concurrently create two writers).
• The PipelinedTreeStoreStrategy doesn't support filtering yet, unlike the regular pipelined strategy.
• TreeStore memory usage: the cache size calculation was wrong: it multiplied by the size factor twice. This could result in out-of-memory.
• The FulltextBinaryTextExtractor didn't properly support concurrent initialization, leading to a NullPointerException.

For the concurrency issues, I added tests. They are pretty fast, because the corruption happens in memory and not on disk.