Fix custom converter params not being thread-safe

[grubeninspekteur]

Issue link

#753 Custom converter param not thread safe

Purpose

see issue

Approach

Thread local param for all instances of CustomConverter.

Open Questions and Pre-Merge TODOs

• Issue created
• Unit tests pass
• Documentation updated
• Travis build passed

[garethahealy]

@idyedov ; can you test this PR to see if it fixes your issue please

[idyedov]

looked over the fix and the included unit test - looks good to me!

longer-term, I would suggest to move parameter to be immutable and set at initialization of the field mapping configuration rather than at runtime

[grubeninspekteur]

@idyedov problem is, the same converter instance may be reused by multiple mappings with different parameters (we do this in our production code). A better solution would be adding the parameter to the mapping method, but this is an incompatible interface change.