MemcacheClient with SerializingTranscoder isn't thread safe

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Version 2.11.2

net.spy.memcached.MemcachedClient.incr(String key, int by, long def, int exp)

sometimes produces this error, when it calls in serveral threads from the 
single volatile MemcachedClient instance. 

java.lang.StringIndexOutOfBoundsException String index out of range
java.lang.String.charAt(String.java:658)
java.util.regex.Pattern$BmpCharProperty.match(Pattern.java:3715)
java.util.regex.Pattern$Curly.match0(Pattern.java:4148)
java.util.regex.Pattern$Curly.match(Pattern.java:4132)
java.util.regex.Pattern$Ques.match(Pattern.java:4079)
java.util.regex.Pattern$Begin.match(Pattern.java:3472)
java.util.regex.Matcher.match(Matcher.java:1221)
java.util.regex.Matcher.matches(Matcher.java:559)
net.spy.memcached.util.StringUtils.isJsonObject(StringUtils.java:113)
net.spy.memcached.transcoders.SerializingTranscoder.encode(SerializingTranscoder
.java:134)
net.spy.memcached.MemcachedClient.asyncStore(MemcachedClient.java:305)
net.spy.memcached.MemcachedClient.asyncStore(MemcachedClient.java:334)
net.spy.memcached.MemcachedClient.mutateWithDefault(MemcachedClient.java:1955)
net.spy.memcached.MemcachedClient.incr(MemcachedClient.java:1902)

The method net.spy.memcached.util.StringUtils.isJsonObject isn't thread safe, 
because it uses the single instance of java.util.regex.Matcher, which isn't 
thread safe.

Original issue reported on code.google.com by igorbun...@issart.com on 15 Dec 2014 at 11:59