Use static factory method to obtain configured object mapper

[nicoweidner]

In particular, we should make sure we don't create another object mapper every time we need one. ObjectMapper instances are supposedly comparatively expansive to create, and if we need to configure it, it would be better to have that configuration extracted.

See https://stackoverflow.com/questions/3907929/should-i-declare-jacksons-objectmapper-as-a-static-field for some suggestions regarding best practices (one global static ObjectMapper may not be the best solution either)