Fix data race.
Fix data race.

This synchronizes access to the (*Client).disconnect channel, which
may change during reconnects.

Fixes #54.

tb00/btcd,runeaune/btcrpcclient,jrick/dcrd,ppcsuite/ppcd,aakselrod/btcd,jrick/btcd,ppcsuite/btcrpcclient,jrick/btcd,davecgh/btcd,runeaune/btcrpcclient,Stash-Crypto/btcd,jrick/btcrpcclient,dajohi/btcd,aakselrod/btcrpcclient,btcsuite/btcd,dajohi/btcd,jrick/dcrd,ltcsuite/ltcd,decred/dcrd,dajohi/btcd,wallclockbuilder/btcd,jrick/btcd,jrick/btcrpcclient,wallclockbuilder/btcd,tb00/btcd,btcsuite/btcrpcclient,btcsuite/btcd,davecgh/btcd,decred/dcrd,wallclockbuilder/btcrpcclient,btcsuite/btcrpcclient,ppcsuite/btcrpcclient,aakselrod/btcrpcclient,Roasbeef/btcd,Roasbeef/btcd,Stash-Crypto/btcd,aakselrod/btcd,ltcsuite/ltcd,wallclockbuilder/btcrpcclient,ppcsuite/ppcd