Rename PICODATA_DATA_DIR to PICODATA_INSTANCE_DIR, admin.sock to admin.socket

Summary

• Config File Changes:

data_dir renamed to instance_dir

• Source Code Changes:

data_dir renamed to instance_dir

admin.sock renamed to admin.socket

• Environment Variable Changes:

PICODATA_DATA_DIR renamed to PICODATA_INSTANCE_DIR

PICODATA_ADMIN_SOCK renamed to PICODATA_ADMIN_SOCKET

• Close #1119
• Cherry-pick to: none
• Docs follow-up: docs#427
• Changelog is updated

Closes #1119