Allow vacuum block to be configured with SQLD_BLOCK_VACUUM env var

[athoscouto]

This would allow libsql-server users to run vacuum locally.

[glommer]

@athoscouto do you plan to rebase this after sarna's comments?

[penberg]

@athoscouto I am closing this because there has been no activity on the pull request for six months and it has tons of conflicts. Please open a new one if you still want to do this.