Memory: poor performance for recursive queries

**Description**
Simple recursive queries take much more compared to their non recursive counterparts.

**Steps to reproduce the issue:**
Follow a simple RDF list
```javascript
g
.V()
.FollowRecursive("")
.Out("")
.All()
```

**Received results:**
Results return 100x more slow

**Expected results:**
Performance to be similar to non recursive queries

**Output of `cayley version` or commit hash:**
https://github.com/cayleygraph/cayley/commit/5318a818e947b20ef36a274f643e577f036bca7c

**Environment details:**
Docker

Backend database:
memory

Start by running the supplied recursive RDF-list query against the memory backend and compare it with the equivalent non-recursive query. Trace the recursive query path to identify where the slowdown occurs; done means the recursive result returns with performance similar to the non-recursive counterpart.

Written by the indexing model from the issue text.