There's a bug with a relevant discussion on systemd's approach to FIFO socket deletion. As of systemd-214 the issue with "stale" sockets was resolved by supplying the RemoveOnStop option to its corresponding .service. However, at the moment RHEL7 has systemd-208 as the default version; and I am seeing the following errors in dmesg output:

systemd[1]: systemd 208 running in system mode. (+PAM +LIBWRAP +AUDIT +SELINUX +IMA +SYSVINIT +LIBCRYPTSETUP +GCRYPT +ACL +XZ)
systemd[1]: /usr/lib/systemd/system-generators/anaconda-generator exited with exit status 1.
[  +0.056775] systemd[1]: [/usr/lib/systemd/system/lvm2-lvmetad.socket:9] Unknown lvalue 'RemoveOnStop' in section 'Socket'
[  +0.000675] systemd[1]: [/usr/lib/systemd/system/dm-event.socket:10] Unknown lvalue 'RemoveOnStop' in section 'Socket'

I'm not sure if LVM2 is referencing a feature that's not available in the default systemd version; AFAIK, my configuration is pretty much standard (default) regarding the pertinent components.

# rpm -qf /usr/lib/systemd/system/lvm2-lvmetad.socket 
lvm2-2.02.115-3.el7.x86_64

# yum info systemd
Installed Packages
Name        : systemd
Arch        : x86_64
Version     : 208
Release     : 20.el7_1.2
Size        : 11 M
Repo        : installed
From repo   : rhel-7-server-rpms