rados_connect not thread-safe when using nss (documentation)

I'm not sure whether rados_connect is expected to be threadsafe or not,
so this is just a documentation patch rather than a fix; I'd appreciate
your opinion on whether this is expected behaviour or not.

The race condition is in the call to ceph::crypto::init when called by
common_init_finish, the issue being that it calls NSS_NoDB_Init (not
threadsafe) without locking. It can be reproduced (probabilistically) by
calling rados_connect on different rados_t objects simultaneously, due
to NSS_NoDB_Init's use of PR_CallOnce in nspr (which keeps global state,
and while PR_CallOnce is intended as a locking function, the locking
itself isn't thread-safe, and can pass PR_Lock a null pointer).

The observed behaviour is a segfault on calling rados_connect.
Backtrace, for reference:

    secmodName=secmodName@entry=0x7ffff4f64817 "", updateDir=updateDir@entry=0x7ffff4f64817 "", updCertPrefix=updCertPrefix@entry=0x7ffff4f64817 "",
    updKeyPrefix=updKeyPrefix@entry=0x7ffff4f64817 "", updateID=updateID@entry=0x7ffff4f64817 "", updateName=updateName@entry=0x7ffff4f64817 "",
    initContextPtr=initContextPtr@entry=0x0, initParams=initParams@entry=0x0, readOnly=readOnly@entry=1, noCertDB=noCertDB@entry=1, noModDB=noModDB@entry=1,
    forceOpen=forceOpen@entry=1, noRootInit=noRootInit@entry=1, optimizeSpace=optimizeSpace@entry=1, noSingleThreadedModules=noSingleThreadedModules@entry=0,
    allowAlreadyInitializedModules=allowAlreadyInitializedModules@entry=0, dontFinalizeModules=dontFinalizeModules@entry=0) at nssinit.c:551

Signed-off-by: Sharif Olorin <sio@tesser.org>
Merge pull request #1424 from fractalcat/thread-safety-doc-update

rados_connect not thread-safe when using nss (documentation)

Reviewed-by: Sage Weil <sage@inktank.com>