fix: make sure use of non-threadsafe UdfIndex is synchronized

[purplefox]

Description

UdfIndex is not thread-safe, and is used in two place in the codebase where it could be accessed by different threads: UdfFactory and UdfAggregateFunctionFactory.

This PR adds synchronization to UdfFactory and `UdfAggregateFunctionFactory' to make this access thread-safe.

Currently addFunction is always called (in clock time) before getFunction. (Although this access pattern is not enforced in the class).

Note that this doesn't make the access safe, because there is no obvious happens-before relationship between the actions on a thread calling addFunction and another thread calling getFunction.

There could be an accidental happens-before due to the way threads are currently started in the server but relying on this is fragile. By synchronizing we guarantee a happens-before relationship between threads accessing these functions.

Testing done

Non functional change

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[agavra]

LGTM!