Add support to ignore MongoDB handshaking frames

[kpattaswamy]

Summary: When end to end testing MongoDB tracing, the parser encountered OP_MSG handshaking frames and was not able to parse them. This was due to the old parser searching for a CRUD command in the OP_MSG frame's top level key in the payload and OP_MSG handshaking frames not containing a key the old parser could interpret.

This PR adds support to parse top level handshaking keys and discards those frames at stitching time. It also correctly prefixes const variables in the types.h file.

The motivation to parse these frames instead of immediately ignoring them at parsing time is to account for the different possibilities of responses for a handshaking request. A handshaking request could lead to a more_to_come response where each frame in the more_to_come response may not be identifiable as a handshaking frame. It's also possible that the response of a handshaking request may contain an ok key which can be misidentified as a non handshaking response and left stale in the map of response deques. Identifying all of the handshaking request/response frames at stitching time but not inserting it to the records would ensure that all handshaking frames are cleared from the map and not pushed to the data table.

Related issues: #640

Type of change: /kind bug

Test Plan: Added a stitcher test

[ddelnano]

Two minor comments but otherwise lgtm.