dolphin: Add function signatures (#796)

* dolphin: Add a subset of math func signatures

* dolphin: Add FROM_UNIXTIME signature

Fixes #655

* dolphin: flow control function signatures

* dolphin: Add ISNULL function signature

Fixes #630

* dolphin: Add STRCMP function signature

* dolphin: Add aggregate function signatures

* dolphin: Add all JSON function signatures

* dolphin: Add signatures for mathematical functions

* dolphin: Add string function signatures

* dolphin: Add date and time function signatures

* dolphin: Add comparison operator function signatures

* dolphin: Add encryption function signatures

* dolphin: Add miscellaneous function signatures

* dolphin: Add information function signatures

* dolphin: Add window function signatures

* dolphin: Add performance schema function signatures

* dolphin: Add locking function signatures

* dolphin: Add regexp function signatures

* dolphin: Add XML function signatures

* dolphin: Add GTID function signatures

* dolphin: Add a few more function signatures

* dolphin: Add spatial analysis function signatures

* dolphin: Remove two unknown functions