[CBM] tmpdir never deleted due to forward slash in base64 encoding

What is the problem?
For the temporary directory we added as part of MB-53226 we create a directory of the form <pid>-<4 chars of hostname hash> so we can quickly decide whether we need to remove a directory. Unfortunately the default base64 alphabet includes forward slashes.

For machines whose first four base64 encoding of their hash has a forward slash this means their temporary directory will never be deleted.

This was found on this Jenkins run.

What is the user impact?
In normal operation there should be near none - the temporary directory will exist but will not take up disk space as SQLite will have cleaned up its temporary files. If a backup or restore is interrupted at the right point files may be left over however.

What is the solution?
We should use the URL encoding as this is also safe for file paths.