[WIP] Consistently use convert_bv instead of convert

[tautschnig]

This will place all expressions in boolbvt's cache instead of sometimes
using prop_convt's cache.

This is factored out from #2033 and needs data to confirm actual improvements or else will just be discarded. No need to review until data is available.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[TGWDB]

Closing as this is a draft PR that has been untouched for a very long time. Please reopen if you update/rebase this.