Add initial block download script

Summary:
This script performs an initial block download, and then
terminates the bitcoind process.

Test Plan:
Run ./contrib/teamcity/ibd.sh from a directory containing a
recently build binary of bitcoind.

Reviewers: deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D994