Update LogMergePolicy to skip to a target number of documents

[kryesh]

From discord conversation:
https://discord.com/channels/908281611840282624/915785344396439552/1341705839668625468

This updated logic makes LogMergePolicy aim for a specific target number of documents, and opportunistically skip merge operations to reach that target document count.
Pros:

• Reduced IO/CPU usage from skipping intermediate merge operations
• No longer susceptible to creating huge merge operations by merging many large segments into a single segment many times larger than the target size (previously max_docs_before_merge)
  • The theoretical maximum size of a segment with this updated logic is (target_segment_size * 2) - 2

Cons:

• If an index has a little over target_segment_size total docs then it may get merged to a single segment and thus not parallelize well when searching