Fix Search reindexing performance regression

[aduffeck]

We've fixed a regression in the search service reindexing step, causing the whole space to be reindexed instead of just the changed resources.

Stats from testing (this was with a really small space, the effects for larger spaces will be more dramatic):

Upload file into the space root with 24 files and 2 subdirs (warm cache):
Before:                                             After:
+-----------+------------------+-------+            +-----------+------------------+-------+
| Function  | last 5 second(s) | total |            | Function  | last 5 second(s) | total |
+-----------+------------------+-------+            +-----------+------------------+-------+
| Chtimes   |                1 |    11 |            | Chtimes   |                1 |    14 |
| Create    |                0 |     5 |            | Create    |                0 |     5 |
| Open      |               13 |    46 |            | Open      |               12 |   100 |
| OpenFile  |                0 |     0 |            | OpenFile  |                0 |     0 |
| Lstat     |              117 |   229 |            | Lstat     |               59 |   476 |
| MkdirAll  |                3 |    26 |            | MkdirAll  |                3 |    43 |
| ReadFile  |                2 |     7 |            | ReadFile  |                2 |    13 |
| Readlink  |               99 |   255 |            | Readlink  |                8 |   206 |
| Rename    |                0 |     0 |            | Rename    |                0 |     8 |
| Remove    |                2 |    22 |            | Remove    |                2 |    36 |
| RemoveAll |                0 |     0 |            | RemoveAll |                0 |     0 |
| Stat      |              110 |   187 |            | Stat      |               12 |   104 |
| Symlink   |                4 |    19 |            | Symlink   |                4 |    35 |
| WriteFile |                0 |     0 |            | WriteFile |                0 |     0 |
+-----------+------------------+-------+            +-----------+------------------+-------+

[sonarcloud]

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