Rename message/API: issueAssetType -> issueInstrumentDefinition.

find . | egrep "(cpp|hpp|proto)$" | xargs sed -i -e 's/issueAssetType/issueInstrumentDefinition/g'
find . | egrep "(cpp|hpp|proto)$" | xargs sed -i -e 's/IssueAssetType/IssueInstrumentDefinition/g'
Merge pull request #454 from benma/renamings

Rename message/API: issueAssetType -> issueInstrumentDefinition.