Perform all request to db in a single thread to avoid blocking navmesh
generator threads due to slow write operations.
Write to db navmesh for all changes except update as it done for memory cache.
Batch multiple db operations into a single transaction to speed up writing by
not executing fsync after each insert/update query. All reads are performed in
the same transaction so they see uncommited data.
To reduce cache size and make it more flexible.
Adding off mesh connections to the navmesh is the last step of navmesh
generation and it's very fast comparing to other steps (microseconds vs
milliseconds). Having less cache size makes get and set operations almost 2x
times faster that also have an order of microseconds. So in total there is
no performance impact.