[lldb-server] Add setting to force 'g' packet use

Following up on https://reviews.llvm.org/D62221, this change introduces
the settings plugin.process.gdb-remote.use-g-packet-for-reading.  When
they are on, 'g' packets are used for reading registers.

Using 'g' packets can improve performance by reducing the number of
packets exchanged between client and server when a large number of
registers needs to be fetched.

Differential revision: https://reviews.llvm.org/D62931