[hack] rename sqlite3 *db to g_db
[hack] rename sqlite3 *db to g_db

Summary:
rename db to g_db

db is used as a parameter name in create_sqlite_header and
verify_sqlite_header.

This makes it more obvious whether a function is using the once-per-runtime
db or a parameter passed in.

Reviewed By: alexchow

Differential Revision: D7265793

fbshipit-source-id: cb3fddc654b00321be16801f13902d5d0857fbd5

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