Improve performance of index operations

[madadam]

Currently the operations on the index (BranchData::get, BranchData::insert and BranchData::remove) are quite expensive. Things are especially bad if multiple such operations need to be executed within a db transaction (for example in fork) because then we hold the db connection until all of the operations are complete which blocks other parts of the system that also need db connection.

We should first create some benchmarks (perhaps using criterion) and then explore ways to optimize this code.

[madadam]

Related: #131

[madadam]

Done in a6fe1d8